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Marco GOVONI

Ricercatore t.d. art. 24 c. 3 lett. B
Dipartimento di Scienze Fisiche, Informatiche e Matematiche sede ex-Fisica

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Pubblicazioni

2023 - Excited State Properties of Point Defects in Semiconductors and Insulators Investigated with Time-Dependent Density Functional Theory [Articolo su rivista]
Jin, Yu; Yu, Victor Wen-Zhe; Govoni, Marco; Xu, Andrew C; Galli, Giulia
abstract

: We present a formulation of spin-conserving and spin-flip hybrid time-dependent density functional theory (TDDFT), including the calculation of analytical forces, which allows for efficient calculations of excited state properties of solid-state systems with hundreds to thousands of atoms. We discuss an implementation on both GPU- and CPU-based architectures along with several acceleration techniques. We then apply our formulation to the study of several point defects in semiconductors and insulators, specifically the negatively charged nitrogen-vacancy and neutral silicon-vacancy centers in diamond, the neutral divacancy center in 4H silicon carbide, and the neutral oxygen-vacancy center in magnesium oxide. Our results highlight the importance of taking into account structural relaxations in excited states in order to interpret and predict optical absorption and emission mechanisms in spin defects.

2023 - Nonempirical Range-Separated Hybrid Functional with Spatially Dependent Screened Exchange [Articolo su rivista]
Zhan, Jiawei; Govoni, Marco; Galli, Giulia
abstract

Electronic structure calculations based on density functionaltheory(DFT) have successfully predicted numerous ground-state propertiesof a variety of molecules and materials. However, exchange and correlationfunctionals currently used in the literature, including semilocaland hybrid functionals, are often inaccurate to describe the electronicproperties of heterogeneous solids, especially systems composed ofbuilding blocks with large dielectric mismatch. Here, we present adielectric-dependent range-separated hybrid functional, screened-exchangerange-separated hybrid (SE-RSH), for the investigation of heterogeneousmaterials. We define a spatially dependent fraction of exact exchangeinspired by the static Coulomb-hole and screened-exchange (COHSEX)approximation used in many-body perturbation theory, and we show thatthe proposed functional accurately predicts the electronic structureof several nonmetallic interfaces, three- and two-dimensional, pristine,and defective solids and nanoparticles.

2023 - Quantum Simulations of Fermionic Hamiltonians with Efficient Encoding and Ansatz Schemes [Articolo su rivista]
Huang, Bc; Sheng, N; Govoni, M; Galli, G
abstract

We propose a computational protocol for quantum simulations of fermionic Hamiltonians on a quantum computer, enabling calculations on spin defect systems which were previously not feasible using conventional encodings and a unitary coupled-cluster ansatz of variational quantum eigensolvers. We combine a qubit-efficient encoding scheme mapping Slater determinants onto qubits with a modified qubitcoupled cluster ansatz and noise-mitigation techniques. Our strategy leads to a substantial improvement in the scaling of circuit gate counts and in the number of required qubits, and to a decrease in the number of required variational parameters, thus increasing the resilience to noise. We present results for spin defects of interest for quantum technologies, going beyond minimum models for the negatively charged nitrogen vacancy center in diamonds and the double vacancy in 4H silicon carbide (4H-SiC) and tackling a defect as complex as negatively charged silicon vacancy in 4H-SiC for the first time.

2023 - Roadmap on electronic structure codes in the exascale era [Articolo su rivista]
Gavini, Vikram; Baroni, Stefano; Blum, Volker; Bowler, David R; Buccheri, Alexander; Chelikowsky, James R; Das, Sambit; Dawson, William; Delugas, Pietro; Dogan, Mehmet; Draxl, Claudia; Galli, Giulia; Genovese, Luigi; Giannozzi, Paolo; Giantomassi, Matteo; Gonze, Xavier; Govoni, Marco; Gygi, François; Gulans, Andris; Herbert, John M; Kokott, Sebastian; Kühne, Thomas D; Liou, Kai-Hsin; Miyazaki, Tsuyoshi; Motamarri, Phani; Nakata, Ayako; Pask, John E; Plessl, Christian; Ratcliff, Laura E; Richard, Ryan M; Rossi, Mariana; Schade, Robert; Scheffler, Matthias; Schütt, Ole; Suryanarayana, Phanish; Torrent, Marc; Truflandier, Lionel; Windus, Theresa L; Xu, Qimen; Yu, Victor W-Z; Perez, D
abstract

Electronic structure calculations have been instrumental in providing many important insights into a range of physical and chemical properties of various molecular and solid-state systems. Their importance to various fields, including materials science, chemical sciences, computational chemistry, and device physics, is underscored by the large fraction of available public supercomputing resources devoted to these calculations. As we enter the exascale era, exciting new opportunities to increase simulation numbers, sizes, and accuracies present themselves. In order to realize these promises, the community of electronic structure software developers will however first have to tackle a number of challenges pertaining to the efficient use of new architectures that will rely heavily on massive parallelism and hardware accelerators. This roadmap provides a broad overview of the state-of-the-art in electronic structure calculations and of the various new directions being pursued by the community. It covers 14 electronic structure codes, presenting their current status, their development priorities over the next five years, and their plans towards tackling the challenges and leveraging the opportunities presented by the advent of exascale computing.

2022 - Computational Protocol to Evaluate Electron-Phonon Interactions Within Density Matrix Perturbation Theory [Articolo su rivista]
Yang, Han; Govoni, Marco; Kundu, Arpan; Galli, Giulia
abstract

: We present a computational protocol, based on density matrix perturbation theory, to obtain non-adiabatic, frequency-dependent electron-phonon self-energies for molecules and solids. Our approach enables the evaluation of electron-phonon interaction using hybrid functionals, for spin-polarized systems, and the computational overhead to include dynamical and non-adiabatic terms in the evaluation of electron-phonon self-energies is negligible. We discuss results for molecules, as well as pristine and defective solids.

2022 - GPU Acceleration of Large-Scale Full-Frequency GW Calculations [Articolo su rivista]
Yu, Victor Wen-Zhe; Govoni, Marco
abstract

: Many-body perturbation theory is a powerful method to simulate electronic excitations in molecules and materials starting from the output of density functional theory calculations. By implementing the theory efficiently so as to run at scale on the latest leadership high-performance computing systems it is possible to extend the scope of GW calculations. We present a GPU acceleration study of the full-frequency GW method as implemented in the WEST code. Excellent performance is achieved through the use of (i) optimized GPU libraries, e.g., cuFFT and cuBLAS, (ii) a hierarchical parallelization strategy that minimizes CPU-CPU, CPU-GPU, and GPU-GPU data transfer operations, (iii) nonblocking MPI communications that overlap with GPU computations, and (iv) mixed precision in selected portions of the code. A series of performance benchmarks has been carried out on leadership high-performance computing systems, showing a substantial speedup of the GPU-accelerated version of WEST with respect to its CPU version. Good strong and weak scaling is demonstrated using up to 25 920 GPUs. Finally, we showcase the capability of the GPU version of WEST for large-scale, full-frequency GW calculations of realistic systems, e.g., a nanostructure, an interface, and a defect, comprising up to 10 368 valence electrons.

2022 - Green's Function Formulation of Quantum Defect Embedding Theory [Articolo su rivista]
Sheng, Nan; Vorwerk, Christian; Govoni, Marco; Galli, Giulia
abstract

: We present a Green's function formulation of the quantum defect embedding theory (QDET) where a double counting scheme is rigorously derived within the G0W0 approximation. We then show the robustness of our methodology by applying the theory with the newly derived scheme to several defects in diamond. Additionally, we discuss a strategy to obtain converged results as a function of the size and composition of the active space. Our results show that QDET is a promising approach to investigate strongly correlated states of defects in solids.

2022 - Quantum embedding theories to simulate condensed systems on quantum computers [Articolo su rivista]
Vorwerk, Christian; Sheng, Nan; Govoni, Marco; Huang, Benchen; Galli, Giulia
abstract

Quantum computers hold promise to improve the efficiency of quantum simulations of materials and to enable the investigation of systems and properties that are more complex than tractable at present on classical architectures. Here, we discuss computational frameworks to carry out electronic structure calculations of solids on noisy intermediate-scale quantum computers using embedding theories, and we give examples for a specific class of materials, that is, solid materials hosting spin defects. These are promising systems to build future quantum technologies, such as quantum computers, quantum sensors and quantum communication devices. Although quantum simulations on quantum architectures are in their infancy, promising results for realistic systems appear to be within reach.

2022 - Simulating the Electronic Structure of Spin Defects on Quantum Computers [Articolo su rivista]
Huang, Benchen; Govoni, Marco; Galli, Giulia
abstract

We present calculations of both the ground- and excited-state energies of spin defects in solids carried out on a quantum computer, using a hybrid classical-quantum protocol. We focus on the negatively charged nitrogen-vacancy center in diamond and on the double vacancy in 4H SiC, which are of interest for the realization of quantum technologies. We employ a recently developed first-principles quantum embedding theory to describe point defects embedded in a periodic crystal and to derive an effective Hamiltonian, which is then transformed to a qubit Hamiltonian by means of a parity transformation. We use the variational quantum eigensolver (VQE) and quantum subspace expansion methods to obtain the ground and excited states of spin qubits, respectively, and we propose a promising strategy for noise mitigation. We show that by combining zero-noise extrapolation techniques and constraints on electron occupation to overcome the unphysical-state problem of the VQE algorithm, one can obtain reasonably accurate results on near-term-noisy architectures for ground- and excited-state properties of spin defects.

2022 - Vibrationally resolved optical excitations of the nitrogen-vacancy center in diamond [Articolo su rivista]
Jin, Yu; Govoni, Marco; Galli, Giulia
abstract

A comprehensive description of the optical cycle of spin defects in solids requires the understanding of the electronic and atomistic structure of states with different spin multiplicity, including singlet states which are particularly challenging from a theoretical standpoint. We present a general framework, based on spin-flip time-dependent density function theory, to determine the excited state potential energy surfaces of the many-body singlet states of spin defects; we then predict the vibrationally resolved absorption spectrum between singlet shelving states of a prototypical defect, the nitrogen-vacancy center in diamond. Our results, which are in very good agreement with experiments, provide an interpretation of the measured spectra and reveal the key role of specific phonons in determining absorption processes, and the notable influence of non-adiabatic interactions. The insights gained from our calculations may be useful in defining strategies to improve infrared-absorption-based magnetometry and optical pumping schemes. The theoretical framework developed here is general and applicable to a variety of other spin defects and materials.

2021 - Code interoperability extends the scope of quantum simulations [Articolo su rivista]
Govoni, Marco; Whitmer, Jonathan; de Pablo, Juan; Gygi, Francois; Galli, Giulia
abstract

The functionality of many materials is critically dependent on the integration of dissimilar components and on the interfaces that arise between them. The description of such heterogeneous components requires the development and deployment of first principles methods, coupled to appropriate dynamical descriptions of matter and advanced sampling techniques, in order to capture all the relevant length and time scales of importance to the materials’ performance. It is thus essential to build simple, streamlined computational schemes for the prediction and design of multiple properties of broad classes of materials, by developing interoperable codes which can be efficiently coupled to each other to perform complex tasks. We discuss the use of interoperable codes to simulate the structural and spectroscopic characterization of materials, including chemical reactions for catalysis, the description of defects for quantum information science, and heat and charge transport.

2021 - Combined First-Principles Calculations of Electron-Electron and Electron-Phonon Self-Energies in Condensed Systems [Articolo su rivista]
Yang, Han; Govoni, Marco; Kundu, Arpan; Galli, Giulia
abstract

: We present a method to efficiently combine the computation of electron-electron and electron-phonon self-energies, which enables the evaluation of electron-phonon coupling at the G0W0 level of theory for systems with hundreds of atoms. In addition, our approach, which is a generalization of a method recently proposed for molecules [J. Chem. Theory Comput. 2018, 14, 6269-6275], enables the inclusion of nonadiabatic and temperature effects at no additional computational cost. We present results for diamond and defects in diamond and discuss the importance of numerically accurate G0W0 band structures to obtain robust predictions of zero point renormalization (ZPR) of band gaps, and of the inclusion of nonadiabatic effects to accurately compute the ZPR of defect states in the band gap.

2021 - Machine learning dielectric screening for the simulation of excited state properties of molecules and materials [Articolo su rivista]
Dong, Sijia S; Govoni, Marco; Galli, Giulia
abstract

: Accurate and efficient calculations of absorption spectra of molecules and materials are essential for the understanding and rational design of broad classes of systems. Solving the Bethe-Salpeter equation (BSE) for electron-hole pairs usually yields accurate predictions of absorption spectra, but it is computationally expensive, especially if thermal averages of spectra computed for multiple configurations are required. We present a method based on machine learning to evaluate a key quantity entering the definition of absorption spectra: the dielectric screening. We show that our approach yields a model for the screening that is transferable between multiple configurations sampled during first principles molecular dynamics simulations; hence it leads to a substantial improvement in the efficiency of calculations of finite temperature spectra. We obtained computational gains of one to two orders of magnitude for systems with 50 to 500 atoms, including liquids, solids, nanostructures, and solid/liquid interfaces. Importantly, the models of dielectric screening derived here may be used not only in the solution of the BSE but also in developing functionals for time-dependent density functional theory (TDDFT) calculations of homogeneous and heterogeneous systems. Overall, our work provides a strategy to combine machine learning with electronic structure calculations to accelerate first principles simulations of excited-state properties.

2021 - OPTIMADE, an API for exchanging materials data [Articolo su rivista]
Andersen, Casper W; Armiento, Rickard; Blokhin, Evgeny; Conduit, Gareth J; Dwaraknath, Shyam; Evans, Matthew L; Fekete, Ádám; Gopakumar, Abhijith; Gražulis, Saulius; Merkys, Andrius; Mohamed, Fawzi; Oses, Corey; Pizzi, Giovanni; Rignanese, Gian-Marco; Scheidgen, Markus; Talirz, Leopold; Toher, Cormac; Winston, Donald; Aversa, Rossella; Choudhary, Kamal; Colinet, Pauline; Curtarolo, Stefano; Di Stefano, Davide; Draxl, Claudia; Er, Suleyman; Esters, Marco; Fornari, Marco; Giantomassi, Matteo; Govoni, Marco; Hautier, Geoffroy; Hegde, Vinay; Horton, Matthew K; Huck, Patrick; Huhs, Georg; Hummelshøj, Jens; Kariryaa, Ankit; Kozinsky, Boris; Kumbhar, Snehal; Liu, Mohan; Marzari, Nicola; Morris, Andrew J; Mostofi, Arash A; Persson, Kristin A; Petretto, Guido; Purcell, Thomas; Ricci, Francesco; Rose, Frisco; Scheffler, Matthias; Speckhard, Daniel; Uhrin, Martin; Vaitkus, Antanas; Villars, Pierre; Waroquiers, David; Wolverton, Chris; Wu, Michael; Yang, Xiaoyu
abstract

: The Open Databases Integration for Materials Design (OPTIMADE) consortium has designed a universal application programming interface (API) to make materials databases accessible and interoperable. We outline the first stable release of the specification, v1.0, which is already supported by many leading databases and several software packages. We illustrate the advantages of the OPTIMADE API through worked examples on each of the public materials databases that support the full API specification.

2021 - Photoluminescence spectra of point defects in semiconductors: Validation of first-principles calculations [Articolo su rivista]
Jin, Yu; Govoni, Marco; Wolfowicz, Gary; Sullivan, Sean E.; Joseph Heremans, F.; Awschalom, David D.; Galli, Giulia
abstract

Optically and magnetically active point defects in semiconductors are interesting platforms for the development of solid state quantum technologies. Their optical properties are usually probed by measuring photoluminescence spectra, which provide information on excitation energies and on the interaction of electrons with lattice vibrations. We present a combined computational and experimental study of photoluminescence spectra of defects in diamond and SiC, aimed at assessing the validity of theoretical and numerical approximations used in first-principles calculations, including the use of the Franck-Condon principle and the displaced harmonic oscillator approximation. We focus on prototypical examples of solid state qubits, the divacancy centers in SiC and the nitrogen-vacancy in diamond, and we report computed photoluminescence spectra as a function of temperature that are in very good agreement with the measured ones. As expected we find that the use of hybrid functionals leads to more accurate results than semilocal functionals. Interestingly our calculations show that constrained density functional theory (CDFT) and time-dependent hybrid DFT perform equally well in describing the excited state potential energy surface of triplet states; our findings indicate that CDFT, a relatively cheap computational approach, is sufficiently accurate for the calculations of photoluminescence spectra of the defects studied here. Finally, we find that only by correcting for finite-size effects and extrapolating to the dilute limit can one obtain a good agreement between theory and experiment. Our results provide a detailed validation protocol of first-principles calculations of photoluminescence spectra, necessary both for the interpretation of experiments and for robust predictions of the electronic properties of point defects in semiconductors.

2021 - Quantum Embedding Theory for Strongly Correlated States in Materials [Articolo su rivista]
Ma, He; Sheng, Nan; Govoni, Marco; Galli, Giulia
abstract

: Quantum embedding theories are promising approaches to investigate strongly correlated electronic states of active regions of large-scale molecular or condensed systems. Notable examples are spin defects in semiconductors and insulators. We present a detailed derivation of a quantum embedding theory recently introduced, which is based on the definition of effective Hamiltonians. The effect of the environment on a chosen active space is accounted for through screened Coulomb interactions evaluated using density functional theory. Importantly, the random phase approximation is not required, and the evaluation of virtual electronic orbitals is circumvented with algorithms previously developed in the context of calculations based on many-body perturbation theory. In addition, we generalize the quantum embedding theory to active spaces composed of orbitals that are not eigenstates of Kohn-Sham Hamiltonians. Finally, we report results for spin defects in semiconductors.

2021 - Quantum vibronic effects on the electronic properties of solid and molecular carbon [Articolo su rivista]
Kundu, Arpan; Govoni, Marco; Yang, Han; Ceriotti, Michele; Gygi, Francois; Galli, Giulia
abstract

We study the effect of quantum vibronic coupling on the electronic properties of carbon allotropes, including molecules and solids, by combining path integral first principles molecular dynamics (FPMD) with a colored noise thermostat. In addition to avoiding several approximations commonly adopted in calculations of electron-phonon coupling, our approach only adds a moderate computational cost to FPMD simulations and hence it is applicable to large supercells, such as those required to describe amorphous solids. We predict the effect of electron-phonon coupling on the fundamental gap of amorphous carbon, and we show that in diamond the zero-phonon renormalization of the band gap is larger than previously reported.

2020 - Correction: A finite-field approach for GW calculations beyond the random phase approximation (Journal of Chemical Theory and Computation (2019) 15:1 (154-164) DOI: 10.1021/acs.jctc.8b00864) [Articolo su rivista]
Ma, He; Govoni, Marco; Gygi, Francois; Galli, Giulia
abstract

2020 - First-principles studies of strongly correlated states in defect spin qubits in diamond [Articolo su rivista]
Ma, He; Sheng, Nan; Govoni, Marco; Galli, Giulia
abstract

: Using a recently developed quantum embedding theory, we present first-principles calculations of strongly correlated states of spin defects in diamond. Using this theory, effective Hamiltonians are constructed, which can be solved by classical and quantum computers; the latter promise a much more favorable scaling as a function of system size than the former. In particular, we report a study on the neutral group-IV vacancy complexes in diamond, and we discuss their strongly correlated spin-singlet and spin-triplet excited states. Our results provide valuable predictions for experiments aimed at optical manipulation of these defects for quantum information technology applications.

2020 - MatD^3^: A Database and Online Presentation Package for Research Data Supporting Materials Discovery, Design, and Dissemination [Articolo su rivista]
Laasner, Raul; Du, Xiaochen; Tanikanti, Aditya; Clayton, Connor; Govoni, Marco; Galli, Giulia; Ropo, Matti; Blum, Volker
abstract

2020 - PyCDFT: A Python package for constrained density functional theory [Articolo su rivista]
Ma, He; Wang, Wennie; Kim, Siyoung; Cheng, Man-Hin; Govoni, Marco; Galli, Giulia
abstract

: We present PyCDFT, a Python package to compute diabatic states using constrained density functional theory (CDFT). PyCDFT provides an object-oriented, customizable implementation of CDFT, and allows for both single-point self-consistent-field calculations and geometry optimizations. PyCDFT is designed to interface with existing density functional theory (DFT) codes to perform CDFT calculations where constraint potentials are added to the Kohn-Sham Hamiltonian. Here, we demonstrate the use of PyCDFT by performing calculations with a massively parallel first-principles molecular dynamics code, Qbox, and we benchmark its accuracy by computing the electronic coupling between diabatic states for a set of organic molecules. We show that PyCDFT yields results in agreement with existing implementations and is a robust and flexible package for performing CDFT calculations. The program is available at https://dx.doi.org/10.5281/zenodo.3821097.

2020 - PyZFS: A Python package for first-principles calculations of zero-field splitting tensors [Articolo su rivista]
Ma, He; Govoni, Marco; Galli, Giulia
abstract

2020 - Quantum simulations of materials on near-term quantum computers [Articolo su rivista]
Ma, He; Govoni, Marco; Galli, Giulia
abstract

Quantum computers hold promise to enable efficient simulations of the properties of molecules and materials; however, at present they only permit ab initio calculations of a few atoms, due to a limited number of qubits. In order to harness the power of near-term quantum computers for simulations of larger systems, it is desirable to develop hybrid quantum-classical methods where the quantum computation is restricted to a small portion of the system. This is of particular relevance for molecules and solids where an active region requires a higher level of theoretical accuracy than its environment. Here, we present a quantum embedding theory for the calculation of strongly-correlated electronic states of active regions, with the rest of the system described within density functional theory. We demonstrate the accuracy and effectiveness of the approach by investigating several defect quantum bits in semiconductors that are of great interest for quantum information technologies. We perform calculations on quantum computers and show that they yield results in agreement with those obtained with exact diagonalization on classical architectures, paving the way to simulations of realistic materials on near-term quantum computers.

2019 - A Finite-Field Approach for GW Calculations beyond the Random Phase Approximation [Articolo su rivista]
Ma, He; Govoni, Marco; Gygi, Francois; Galli, Giulia
abstract

We describe a finite-field approach to compute density response functions, which allows for efficient G0W0 and G0W0Γ0 calculations beyond the random phase approximation. The method is easily applicable to density functional calculations performed with hybrid functionals. We present results for the electronic properties of molecules and solids, and we discuss a general scheme to overcome slow convergence of quasiparticle energies obtained from G0W0Γ0 calculations, as a function of the basis set used to represent the dielectric matrix.

2019 - Dielectric-dependent hybrid functionals for heterogeneous materials [Articolo su rivista]
Zheng, Huihuo; Govoni, Marco; Galli, Giulia
abstract

We derive a dielectric-dependent hybrid functional which accurately describes the electronic properties of heterogeneous interfaces and surfaces, as well as those of three- and two-dimensional bulk solids. The functional, which does not contain any adjustable parameter, is a generalization of self-consistent hybrid functionals introduced for homogeneous solids, where the screened Coulomb interaction is defined using a spatially varying, local dielectric function. The latter is determined self-consistently using density functional calculations in finite electric fields. We present results for the band gaps and dielectric constants of 3D and 2D bulk materials, and band offsets for interfaces, showing an accuracy comparable to that of GW calculations.

2019 - Finite-Field Approach to Solving the Bethe-Salpeter Equation [Articolo su rivista]
Nguyen, Ngoc Linh; Ma, He; Govoni, Marco; Gygi, Francois; Galli, Giulia
abstract

: We present a method to compute optical spectra and exciton binding energies of molecules and solids based on the solution of the Bethe-Salpeter equation and the calculation of the screened Coulomb interaction in a finite field. The method does not require either the explicit evaluation of dielectric matrices or of virtual electronic states, and can be easily applied without resorting to the random phase approximation. In addition, it utilizes localized orbitals obtained from Bloch states using bisection techniques, thus greatly reducing the complexity of the calculation and enabling the efficient use of hybrid functionals to obtain single particle wave functions. We report exciton binding energies of several molecules and absorption spectra of condensed systems of unprecedented size, including water and ice samples with hundreds of atoms.

2019 - Improving the efficiency of G0W0 calculations with approximate spectral decompositions of dielectric matrices [Articolo su rivista]
Yang, Han; Govoni, Marco; Galli, Giulia
abstract

: Recently, it was shown that the calculation of quasiparticle energies using the G0W0 approximation can be performed without computing explicitly any virtual electronic states, by expanding the Green function and screened Coulomb interaction in terms of the eigenstates of the static dielectric matrix. Avoiding the evaluation of virtual electronic states leads to improved efficiency and ease of convergence of G0W0 calculations. Here, we propose a further improvement of the efficiency of these calculations, based on an approximation of density-density response functions of molecules and solids. The approximation relies on the calculation of a subset of eigenvectors of the dielectric matrix using the kinetic operator instead of the full Hamiltonian, and it does not lead to any substantial loss of accuracy for the quasiparticle energies. The computational savings introduced by this approximation depend on the system, and they become more substantial as the number of electrons increases.

2019 - Qresp, a tool for curating, discovering and exploring reproducible scientific papers [Articolo su rivista]
Govoni, Marco; Munakami, Milson; Tanikanti, Aditya; Skone, Jonathan H; Runesha, Hakizumwami B; Giberti, Federico; de Pablo, Juan; Galli, Giulia
abstract

: We propose a strategy and present a simple tool to facilitate scientific data reproducibility by making available, in a distributed manner, all data and procedures presented in scientific papers, together with metadata to render them searchable and discoverable. In particular, we describe a graphical user interface (GUI), Qresp, to curate papers (i.e. generate metadata) and to explore curated papers and automatically access the data presented in scientific publications.

2018 - Communication: Dielectric properties of condensed systems composed of fragments [Articolo su rivista]
Pan, Ding; Govoni, Marco; Galli, Giulia
abstract

: The dielectric properties of molecules and nanostructures are usually modified in a complex manner, when assembled into a condensed phase. We propose a first-principles method to compute polarizabilities of sub-entities of solids and liquids, which accounts for multipolar interactions at all orders and is applicable to semiconductors and insulators. The method only requires the evaluation of induced fields in the condensed phase, with no need of multiple calculations for each constituent. As an example, we present results for the molecular polarizabilities of water in a wide pressure and temperature range. We found that at ambient conditions, the dipole-induced-dipole approximation is sufficiently accurate and the Clausius-Mossotti relation may be used, e.g., to obtain molecular polarizabilities from experimental refractive indexes. However with increasing pressure, this approximation becomes unreliable and in the case of ice X the Clausius-Mossotti relation is not valid.

2018 - Coupling First-Principles Calculations of Electron-Electron and Electron-Phonon Scattering, and Applications to Carbon-Based Nanostructures [Articolo su rivista]
Mcavoy, Ryan L; Govoni, Marco; Galli, Giulia
abstract

: We report first-principles calculations of electronic gaps, lifetimes, and photoelectron spectra of a series of molecules, performed by efficiently combining the computation of electron-electron and electron-phonon self-energies. The dielectric matrix is represented in terms of dielectric eigenpotentials, utilized for both the calculation of G0 W0 quasi-particle energies and the diagonalization of the dynamical matrix; virtual electronic states are never explicitly computed and all self-energies are evaluated over the full frequency spectrum. Our formulation enables electronic structure calculations at the many-body perturbation theory level, inclusive of electron-phonon coupling, for systems with hundreds of electrons.

2018 - Electron affinity of liquid water [Articolo su rivista]
Gaiduk, Alex P; Pham, Tuan Anh; Govoni, Marco; Paesani, Francesco; Galli, Giulia
abstract

: Understanding redox and photochemical reactions in aqueous environments requires a precise knowledge of the ionization potential and electron affinity of liquid water. The former has been measured, but not the latter. We predict the electron affinity of liquid water and of its surface from first principles, coupling path-integral molecular dynamics with ab initio potentials, and many-body perturbation theory. Our results for the surface (0.8 eV) agree well with recent pump-probe spectroscopy measurements on amorphous ice. Those for the bulk (0.1-0.3 eV) differ from several estimates adopted in the literature, which we critically revisit. We show that the ionization potential of the bulk and surface are almost identical; instead their electron affinities differ substantially, with the conduction band edge of the surface much deeper in energy than that of the bulk. We also discuss the significant impact of nuclear quantum effects on the fundamental gap and band edges of the liquid.

2018 - Fundamental principles for calculating charged defect ionization energies in ultrathin two-dimensional materials [Articolo su rivista]
Smart, Tyler J.; Wu, Feng; Govoni, Marco; Ping, Yuan
abstract

Defects in two-dimensional (2D) materials are becoming prominent candidates for quantum emitters and scalable optoelectronic applications. However, several physical properties that characterize their behavior, such as charged defect ionization energies, are difficult to simulate with conventional first-principles methods, mainly because of the weak and anisotropic dielectric screening caused by the reduced dimensionality. We establish fundamental principles for accurate and efficient calculations of charged defect ionization energies and electronic structure in ultrathin 2D materials. We propose to use the vacuum level as the reference for defect charge transition levels (CTLs) because it gives robust results insensitive to the level of theory, unlike commonly used band-edge positions. Furthermore, we determine the fraction of Fock exchange in hybrid functionals for accurate band gaps and band-edge positions of 2D materials by enforcing the generalized Koopmans’ condition of localized defect states. We found that the obtained fractions of Fock exchange vary significantly from 0.2 for bulk hexagonal boron nitride (h-BN) to 0.4 for monolayer h-BN, whose band gaps are also in good agreement with experimental results and calculated GW results. The combination of these methods allows for the reliable and efficient prediction of defect ionization energies (the difference between CTLs and band-edge positions). We motivate and generalize these findings with several examples including different defects in monolayer to few-layer h-BN, monolayer MoS2 , and graphane. Finally, we show that increasing the number of layers of h-BN systematically lowers defect ionization energies, mainly through CTLs shifting towards vacuum, with conduction band minima kept almost unchanged.

2018 - GW100: Comparison of Methods and Accuracy of Results Obtained with the WEST Code [Articolo su rivista]
Govoni, Marco; Galli, Giulia
abstract

: The reproducibility of calculations carried out within many-body perturbation theory at the G0 W0 level is assessed for 100 closed shell molecules and compared to that of density functional theory. We consider vertical ionization potentials (VIP) and electron affinities (VEA) obtained with five different codes: BerkeleyGW, FHI-aims, TURBOMOLE, VASP, and WEST. We review the approximations and parameters that control the accuracy of G0 W0 results in each code, and we discuss in detail the effect of extrapolation techniques for the parameters entering the WEST code. Differences between the VIP and VEA computed with the various codes are within ∼60 and ∼120 meV, respectively, which is up to four times larger than in the case of the best results obtained with DFT codes. Vertical ionization potentials are validated against experiment and CCSD(T) quantum chemistry results showing a mean absolute relative error of ∼4% for data obtained with WEST. Our analysis of the differences between localized orbitals and plane-wave implementations points out molecules containing Cu, I, Ga, and Xe as major sources of discrepancies, which call for a re-evaluation of the pseudopotentials used for these systems in G0 W0 calculations.

2018 - The role of defects and excess surface charges at finite temperature for optimizing oxide photoabsorbers [Articolo su rivista]
Gerosa, Matteo; Gygi, Francois; Govoni, Marco; Galli, Giulia
abstract

: Computational screening of materials for solar to fuel conversion technologies has mostly focused on bulk properties, thus neglecting the structure and chemistry of surfaces and interfaces with water. We report a finite temperature study of WO3, a promising anode for photoelectrochemical cells, carried out using first-principles molecular dynamics simulations coupled with many-body perturbation theory. We identified three major factors determining the chemical reactivity of the material interfaced with water: the presence of surface defects, the dynamics of excess charge at the surface, and finite temperature fluctuations of the surface electronic orbitals. These general descriptors are essential for the understanding and prediction of optimal oxide photoabsorbers for water oxidation.

2017 - Carrier Multiplication in Silicon Nanocrystals: Theoretical Methodologies and Role of the Passivation [Articolo su rivista]
Marri, I.; Govoni, M.; Ossicini, S.
abstract

Carrier multiplication is a non-radiative recombination mechanism that leads to the generation of two or more electron–hole pairs after absorption of a single photon. By reducing the occurrence of dissipative effects, this process can be exploited to increase solar cell performance. In this work, we introduce two different theoretical fully ab initio tools that can be adopted to study carrier multiplication in nanocrystals. The tools are described in detail and compared. Subsequently, we calculate carrier multiplication lifetimes in H- and OH-terminated silicon nanocrystals, pointed out the role played by the passivation on the carrier multiplication processes.

2017 - Designing defect-based qubit candidates in wide-gap binary semiconductors for solid-state quantum technologies [Articolo su rivista]
Seo, Hosung; Ma, He; Govoni, Marco; Galli, Giulia
abstract

The development of novel quantum bits is key to extending the scope of solid-state quantum-information science and technology. Using first-principles calculations, we propose that large metal ion–vacancy pairs are promising qubit candidates in two binary crystals: 4H-SiC and w-AlN. In particular, we found that the formation of neutral Hf- and Zr-vacancy pairs is energetically favorable in both solids; these defects have spin-triplet ground states, with electronic structures similar to those of the diamond nitrogen-vacancy center and the SiC divacancy. Interestingly, they exhibit different spin-strain coupling characteristics, and the nature of heavy metal ions may allow for easy defect implantation in desired lattice locations and ensure stability against defect diffusion. To support future experimental identification of the proposed defects, we report predictions of their optical zero-phonon line, zero-field splitting, and hyperfine parameters. The defect design concept identified here may be generalized to other binary semiconductors to facilitate the exploration of new solid-state qubits.

2017 - Electronic structure of aqueous solutions: Bridging the gap between theory and experiments [Articolo su rivista]
Pham, Tuan Anh; Govoni, Marco; Seidel, Robert; Bradforth, Stephen E; Schwegler, Eric; Galli, Giulia
abstract

: Predicting the electronic properties of aqueous liquids has been a long-standing challenge for quantum mechanical methods. However, it is a crucial step in understanding and predicting the key role played by aqueous solutions and electrolytes in a wide variety of emerging energy and environmental technologies, including battery and photoelectrochemical cell design. We propose an efficient and accurate approach to predict the electronic properties of aqueous solutions, on the basis of the combination of first-principles methods and experimental validation using state-of-the-art spectroscopic measurements. We present results of the photoelectron spectra of a broad range of solvated ions, showing that first-principles molecular dynamics simulations and electronic structure calculations using dielectric hybrid functionals provide a quantitative description of the electronic properties of the solvent and solutes, including excitation energies. The proposed computational framework is general and applicable to other liquids, thereby offering great promise in understanding and engineering solutions and liquid electrolytes for a variety of important energy technologies.

2017 - First-Principles Simulations of Functional Materials for Energy Conversion [Altro]
Williams, Timothy J.; Balakrishnan, Ramesh; Zheng, Huihuo; Knight, Christopher; Govoni, Marco; Galli, Giulia; Gygi, Francois
abstract

Computational modeling has become a very effective approach in predicting properties of materials, and in designing functional materials with targeted structural, thermal, or optical properties. Many electronic structure methods have been developed, including density functional theory. Among them, DFT has been widely used in physics and materials science because it is computationally cheaper than other methods but still gives desired accuracy. It also provides a good starting point for higher levels of theory, such as many-body perturbation theory and quantum Monte Carlo. In parallel to these electronic structure method developments, a dramatic increase in computing capabilities over the last decade has enabled large-scale electronic structure calculations to address leading-edge materials science problems. In particular, with Theta at the Argonne Leadership Computing Facility (ALCF), our early science project investigated large-scale nanostructured ma- terials for energy conversion and storage using two open-source electronic structure codes Qbox (http://qboxcode.org) and WEST (http://west-code.org). Qbox is an ab-initio molecular dynamics code based on plane wave DFT, and WEST is a post-DFT code for excited state calculations within many-body perturbation theory.

2017 - Multiple excitation generation in silicon nanocrystals [Relazione in Atti di Convegno]
Marri, Ivan; Ossicini, Stefano; Govoni, M.
abstract

We present density functional theory calculations of carrier multiplication processes in a system of strongly coupled silicon nanocrystals

2017 - Performance and Self-Consistency of the Generalized Dielectric Dependent Hybrid Functional [Articolo su rivista]
Brawand, Nicholas P; Govoni, Marco; Vörös, Márton; Galli, Giulia
abstract

: We analyze the performance of the recently proposed screened exchange constant functional (SX) ( Brawand et al. Phys. Rev. X 2016 , 6 , 041002 ) on the GW100 test set, and we discuss results obtained at different levels of self-consistency. The SX functional is a generalization of dielectric dependent hybrid functionals to finite systems; it is nonempirical and depends on the average screening of the exchange interaction. We compare results for ionization potentials obtained with SX to those of CCSD(T) calculations and experiments, and we find excellent agreement, on par with recent state of the art methods based on many body perturbation theory. Applying SX perturbatively to correct PBE eigenvalues yields improved results in most cases, except for ionic molecules, for which wave function self-consistency is instead crucial. Calculations where wave functions and the screened exchange constant (αSX) are determined self-consistently, and those where αSX is fixed to the value determined within PBE, yield results of comparable accuracy. Perturbative G0W0 corrections of eigenvalues obtained with self-consistent αSX are small on average, for all molecules in the GW100 test set.

2016 - Design of defect spins in piezoelectric aluminum nitride for solid-state hybrid quantum technologies [Articolo su rivista]
Seo, Hosung; Govoni, Marco; Galli, Giulia
abstract

: Spin defects in wide-band gap semiconductors are promising systems for the realization of quantum bits, or qubits, in solid-state environments. To date, defect qubits have only been realized in materials with strong covalent bonds. Here, we introduce a strain-driven scheme to rationally design defect spins in functional ionic crystals, which may operate as potential qubits. In particular, using a combination of state-of-the-art ab-initio calculations based on hybrid density functional and many-body perturbation theory, we predicted that the negatively charged nitrogen vacancy center in piezoelectric aluminum nitride exhibits spin-triplet ground states under realistic uni- and bi-axial strain conditions; such states may be harnessed for the realization of qubits. The strain-driven strategy adopted here can be readily extended to a wide range of point defects in other wide-band gap semiconductors, paving the way to controlling the spin properties of defects in ionic systems for potential spintronic technologies.

2016 - First-principles calculations of electronic coupling effects in silicon nanocrystals: Influence on near band-edge states and on carrier multiplication processes [Articolo su rivista]
Marri, Ivan; Govoni, Marco; Ossicini, Stefano
abstract

Arrays of closely packed nanocrystals show interesting properties that can be exploited to induce new features in nanostructured optoelectronic devices. In this work we study, by first principles calculations, effects induced on near band-edge states and on carrier multiplication by nanocrystals interplay. By considering both hydrogenated and oxygenated structures, we prove that interaction between silicon nanocrystals can alter both the energy gap of the system and dynamics of excited states with a relevance that depends on the nanocrystal-nanocrystal separation, on nanocrystals orientation and on nanocrystals surface properties.

2016 - Generalization of Dielectric-Dependent Hybrid Functionals to Finite Systems [Articolo su rivista]
Brawand, Nicholas P.; Vörös, Márton; Govoni, Marco; Galli, Giulia
abstract

The accurate prediction of electronic and optical properties of molecules and solids is a persistent challenge for methods based on density functional theory. We propose a generalization of dielectric-dependent hybrid functionals to finite systems where the definition of the mixing fraction of exact and semilocal exchange is physically motivated, nonempirical, and system dependent. The proposed functional yields ionization potentials, and fundamental and optical gaps of many, diverse molecular systems in excellent agreement with experiments, including organic and inorganic molecules and semiconducting nanocrystals. We further demonstrate that this hybrid functional gives the correct alignment between energy levels of the exemplary TTF-TCNQ donor-acceptor system.

2016 - Implementation and Validation of Fully Relativistic GW Calculations: Spin-Orbit Coupling in Molecules, Nanocrystals, and Solids [Articolo su rivista]
Scherpelz, Peter; Govoni, Marco; Hamada, Ikutaro; Galli, Giulia
abstract

: We present an implementation of G0W0 calculations including spin-orbit coupling (SOC) enabling investigations of large systems, with thousands of electrons, and we discuss results for molecules, solids, and nanocrystals. Using a newly developed set of molecules with heavy elements (called GW-SOC81), we find that, when based upon hybrid density functional calculations, fully relativistic (FR) and scalar-relativistic (SR) G0W0 calculations of vertical ionization potentials both yield excellent performance compared to experiment, with errors below 1.9%. We demonstrate that while SR calculations have higher random errors, FR calculations systematically underestimate the VIP by 0.1 to 0.2 eV. We further verify that SOC effects may be well approximated at the FR density functional level and then added to SR G0W0 results for a broad class of systems. We also address the use of different root-finding algorithms for the G0W0 quasiparticle equation and the significant influence of including d electrons in the valence partition of the pseudopotential for G0W0 calculations. Finally, we present statistical analyses of our data, highlighting the importance of separating definitive improvements from those that may occur by chance due to a limited number of samples. We suggest the statistical analyses used here will be useful in the assessment of the accuracy of a large variety of electronic structure methods.

2016 - Nonempirical range-separated hybrid functionals for solids and molecules [Articolo su rivista]
Skone, Jonathan H.; Govoni, Marco; Galli, Giulia
abstract

2016 - Photoelectron Spectra of Aqueous Solutions from First Principles [Articolo su rivista]
Gaiduk, Alex P; Govoni, Marco; Seidel, Robert; Skone, Jonathan H; Winter, Bernd; Galli, Giulia
abstract

: We present a combined computational and experimental study of the photoelectron spectrum of a simple aqueous solution of NaCl. Measurements were conducted on microjets, and first-principles calculations were performed using hybrid functionals and many-body perturbation theory at the G0W0 level, starting with wave functions computed in ab initio molecular dynamics simulations. We show excellent agreement between theory and experiments for the positions of both the solute and solvent excitation energies on an absolute energy scale and for peak intensities. The best comparison was obtained using wave functions obtained with dielectric-dependent self-consistent and range-separated hybrid functionals. Our computational protocol opens the way to accurate, predictive calculations of the electronic properties of electrolytes, of interest to a variety of energy problems.

2016 - Silicon Nanocrystals for Photonics and Photovoltaics: Ab-initio Results [Capitolo/Saggio]
Ossicini, Stefano; Govoni, Marco; Guerra, Roberto; Marri, Ivan
abstract

Review Article on electronic and optical properties of silcon nanocrystals for photonic and photovoltaic applications

2015 - Carrier Multiplication in Isolated and Interacting Silicon Nanocrystals [Capitolo/Saggio]
Marri, Ivan; Ossicini, Stefano; Govoni, M.
abstract

n/a

2015 - Carrier multiplication in silicon nanocrystals: ab initio results [Articolo su rivista]
Marri, Ivan; Govoni, Marco; Ossicini, Stefano
abstract

One of the most important goals in the field of renewable energy is the development of original solar cell schemes employing new materials to overcome the performance limitations of traditional solar cell devices. Among such innovative materials, nanostructures have emerged as an important class of materials that can be used to realize efficient photovoltaic devices. When these systems are implemented into solar cells, new effects can be exploited to maximize the harvest of solar radiation and to minimize the loss factors. In this context, carrier multiplication seems one promising way to minimize the effects induced by thermalization loss processes thereby significantly increasing the solar cell power conversion. In this work we analyze and quantify different types of carrier multiplication decay dynamics by analyzing systems of isolated and coupled silicon nanocrystals. The effects on carrier multiplication dynamics by energy and charge transfer processes are also discussed.

2015 - Large scale GW calculations [Articolo su rivista]
Govoni, Marco; Galli, Giulia
abstract

We present GW calculations of molecules, ordered and disordered solids and interfaces, which employ an efficient contour deformation technique for frequency integration and do not require the explicit evaluation of virtual electronic states nor the inversion of dielectric matrices. We also present a parallel implementation of the algorithm which takes advantage of separable expressions of both the single particle Green's function and the screened Coulomb interaction. The method can be used starting from density functional theory calculations performed with semilocal or hybrid functionals. We applied the newly developed technique to GW calculations of systems of unprecedented size, including water/semiconductor interfaces with thousands of electrons.

2014 - Red-shifted carrier multiplication energy threshold and exciton recycling mechanisms in strongly interacting silicon nanocrystals. [Articolo su rivista]
Marri, Ivan; Govoni, Marco; Ossicini, Stefano
abstract

We present density functional theory calculations of carrier multiplication properties in a system of strongly coupled silicon nanocrystals. Our results suggest that nanocrystal-nanocrystal interaction can lead to a reduction of the carrier multiplication energy threshold without altering the carrier multiplication efficiency at high energies, in agreement with experiments. The time evolution of the number of electron-hole pairs generated in a system of strongly interacting nanocrystals upon absorption of high-energy photons is analyzed by solving a system of coupled rate equations, where exciton recycling mechanisms are implemented. We reconsider the role played by Auger recombination which is here accounted also as an active, nondetrimental process.

2014 - Self-consistent hybrid functional for condensed systems [Articolo su rivista]
Skone, Jonathan H.; Govoni, Marco; Galli, Giulia
abstract

2012 - Carrier multiplication between interacting nanocrystals for fostering silicon-based photovoltaics [Articolo su rivista]
Govoni, Marco; Marri, Ivan; Ossicini, Stefano
abstract

The conversion of solar radiation into electric current with high efficiency is one of the most important topics of modern scientific research, as it holds great potential as a source of clean and renewable energy. Exploitation of interaction between nanocrystals seems to be a promising route to the establishment of third-generation photovoltaics. Here, we adopt a fully ab initio scheme to estimate the role of nanoparticle interplay in the carrier multiplication dynamics of interacting silicon nanocrystals. Energy and charge transfer-based carrier multiplication events are studied as a function of nanocrystal separation, demonstrating the benefits induced by the wavefunction sharing regime. We prove the relevance of these recombinative mechanisms for photovoltaic applications in the case of silicon nanocrystals arranged in dense arrays, quantifying at an atomic scale which conditions maximize the outcome.

2011 - Auger recombination in Si and GaAs semiconductors : Ab initio results [Articolo su rivista]
Govoni, Marco; Marri, Ivan; Ossicini, Stefano
abstract

A detailed description, at the atomistic scale, of the dynamics of excess electrons and holes is fundamentalin order to improve the performance of many optoelectronic devices. Among all recombination processes,nonradiative decay paths play a fundamental role in most semiconductor devices, such as optoelectronic devicesand solar cells, limiting their efficiency. In this work, a precise ab initio analysis of the direct Auger recombinationprocesses in both n- and p-type Si and GaAs crystals is presented. Our simulations of minority carrier Augerlifetimes rely on an accurate electronic band structure, calculated using density functional theory with theinclusion of quasiparticle corrections. The results obtained are in good agreement with experimental data forboth n-Si and p-GaAs, proving the importance of the direct Auger recombination mechanism in such systems. Onthe contrary, we show that different nonradiative recombination paths are necessary to explain the experimentalresults for both p-Si and n-GaAs.

2009 - Role of surface states in the Casimir force between semiconducting films [Relazione in Atti di Convegno]
M., Govoni; CALANDRA BUONAURA, Carlo; A., Benassi
abstract

We present results of first principle calculations of the Casimir force between Si films of nanometric size, which show that it depends dignificantly upon the configuration of the surface atoms and give evidence of the importance of surface states.