Publications (27)90.34 Total impact

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ABSTRACT: The accuracy of the manybody perturbation theory GW formalism to calculate electronphonon coupling matrix elements has been recently demonstrated in the case of a few important systems. However, the related computational costs are high and thus represent strong limitations to its widespread application. In the present study, we explore two less demanding alternatives for the calculation of electronphonon coupling matrix elements on the manybody perturbation theory level. Namely, we test the accuracy of the static Coulombhole plus screenedexchange (COHSEX) approximation and further of the constant screening approach, where variations of the screened Coulomb potential W upon small changes of the atomic positions along the vibrational eigenmodes are neglected. We find this latter approximation to be the most reliable, whereas the static COHSEX ansatz leads to substantial errors. Our conclusions are validated in a few paradigmatic cases: diamond, graphene and the C60 fullerene. These findings open the way for combining the present manybody perturbation approach with efficient linearresponse theories.  [Show abstract] [Hide abstract]
ABSTRACT: Density Functional Theory calculations traditionally suffer from an inherent cubic scaling with respect to the size of the system, making big calculations extremely expensive. This cubic scaling can be avoided by the use of socalled linear scaling algorithms, which have been developed during the last few decades. In this way it becomes possible to perform abinitio calculations for several tens of thousands of atoms or even more within a reasonable time frame. However, even though the use of linear scaling algorithms is physically well justified, their implementation often introduces some small errors. Consequently most implementations offering such a linear complexity either yield only a limited accuracy or, if one wants to go beyond this restriction, require a tedious fine tuning of many parameters. In our linear scaling approach within the BigDFT package, we were able to overcome this restriction. Using an ansatz based on localized support functions expressed in an underlying Daubechies wavelet basis  which offers ideal properties for accurate linear scaling calculations  we obtain an amazingly high accuracy and a universal applicability while still keeping the possibility of simulating large systems with only a moderate demand of computing resources.  [Show abstract] [Hide abstract]
ABSTRACT: We propose to use a blend of methodologies to tackle a challenging case for quantum approaches: the simulation of the optical properties of asymmetric fluoroborate derivatives. Indeed, these dyes, which present a lowlying excitedstate exhibiting a cyaninelike nature, are treated not only with the TimeDependent Density Functional Theory (TDDFT) method to determine both the structures and vibrational patterns but also with the BetheSalpeter approach to compute both the vertical absorption and emission energies. This combination allows us to obtain 00 energies with a significantly improved accuracy compared to the "raw" TDDFT estimates. We also discuss the impact of various declinations of the Polarizable Continuum Model (linearresponse, corrected linearresponse, and statespecific models) on the obtained accuracy. 
Article: Benchmark ManyBody GW and Bethe–Salpeter Calculations for Small Transition Metal Molecules
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ABSTRACT: We study the electronic and optical properties of 39 small molecules containing transition metal atoms and 7 others related to quantumdots for photovoltaics. We explore in particular the merits of the manybody GW formalism, as compared to the Delta SCF approach within density functional theory, in the description of the ionization energy and electronic affinity. Mean average errors of 0.20.3 eV with respect to experiment are found when using the PBE0 functional for Delta SCF and as a starting point for GW. The effect of partial selfconsistency at the GW level is explored. Further, for optical excitations, the BetheSalpeter formalism is found to offer similar accuracy as timedependent DFTbased methods with the hybrid PBE0 functional, with mean average discrepancies of about 0.3 and 0.2 eV, respectively, as compared to available experimental data. Our calculations validate the accuracy of the parameterfree GW and BetheSalpeter formalisms for this class of systems, opening the way to the study of large clusters containing transition metal atoms of interest for photovoltaic applications.  [Show abstract] [Hide abstract]
ABSTRACT: The renormalization of electronic eigenenergies due to electronphonon interactions (temperature dependence and zeropoint motion effect) is important in many materials. We address it in the adiabatic harmonic approximation, based on first principles (e.g. DensityFunctional Theory), from different points of view: directly from atomic position fluctuations or, alternatively, from Janak's theorem generalized to the case where the Helmholtz free energy, including the vibrational entropy, is used. We prove their equivalence, based on the usual form of Janak's theorem and on the dynamical equation. We then also place the AllenHeineCardona (AHC) theory of the renormalization in a firstprinciple context. The AHC theory relies on the rigidion approximation, and naturally leads to a selfenergy (Fan) contribution and a DebyeWaller contribution. Such a splitting can also be done for the complete harmonic adiabatic expression, in which the rigidion approximation is not required. A numerical study within the DensityFunctional Perturbation theory framework allows us to compare the AHC theory with frozenphonon calculations, with or without the rigidion terms. For the two different numerical approaches without rigidion terms, the agreement is better than 7 $\mu$eV in the case of diamond, which represent an agreement to 5 significant digits. The magnitude of the non rigidion terms in this case is also presented, distinguishing specific phonon modes contributions to different electronic eigenenergies.  [Show abstract] [Hide abstract]
ABSTRACT: We compute the zeropoint renormalization (ZPR) of the optical band gap of diamond from manybody perturbation theory using the perturbative ${G}_{0}{W}_{0}$ approximation as well as quasiparticle selfconsistent $GW$. The electronphonon coupling energies are found to be more than 40% higher than standard density functional theory when manybody effects are included with the frozenphonon calculations. A similar increase is observed for the zeropoint renormalization in GaAs when ${G}_{0}{W}_{0}$ corrections are applied. We show that these manybody corrections are necessary to accurately predict the temperature dependence of the band gap. The frozenphonon method also allows us to validate the rigidion approximation which is always present in density functional perturbation theory. 
Article: Fast and Accurate Electronic Excitations in Cyanines with the ManyBody Bethe–Salpeter Approach
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ABSTRACT: The accurate prediction of the optical signatures of cyanine derivatives remains an important challenge in theoretical chemistry. Indeed, up to now, only the most expensive quantum chemical methods (CASPT2, CC, DMC, etc.) yield consistent and accurate data, impeding the applications on reallife molecules. Here, we investigate the lowest lying singlet excitation energies of increasingly long cyanine dyes within the GW and Bethe–Salpeter Green’s function manybody perturbation theory. Our results are in remarkable agreement with available coupledcluster (exCC3) data, bringing these two singlereference perturbation techniques within a 0.05 eV maximum discrepancy. By comparison, available TDDFT calculations with various semilocal, global, or rangeseparated hybrid functionals, overshoot the transition energies by a typical error of 0.3–0.6 eV. The obtained accuracy is achieved with a parameterfree formalism that offers similar accuracy for metallic or insulating, finite size or extended systems.  [Show abstract] [Hide abstract]
ABSTRACT: Manybody Green's function perturbation theories, such as the GW and BetheSalpeter formalisms, are starting to be routinely applied to study charged and neutral electronic excitations in molecular organic systems relevant to applications in photovoltaics, photochemistry or biology. In parallel, density functional theory and its timedependent extensions significantly progressed along the line of rangeseparated hybrid functionals within the generalized KohnSham formalism designed to provide correct excitation energies. We give an overview and compare these approaches with examples drawn from the study of gas phase organic systems such as fullerenes, porphyrins, bacteriochlorophylls or nucleobases molecules. The perspectives and challenges that manybody perturbation theory is facing, such as the role of selfconsistency, the calculation of forces and potential energy surfaces in the excited states, or the development of embedding techniques specific to the GW and BetheSalpeter equation formalisms, are outlined.  [Show abstract] [Hide abstract]
ABSTRACT: We demonstrate that Daubechies wavelets can be used to construct a minimal set of optimized localized contracted basis functions in which the KohnSham orbitals can be represented with an arbitrarily high, controllable precision. Ground state energies and the forces acting on the ions can be calculated in this basis with the same accuracy as if they were calculated directly in a Daubechies wavelets basis, provided that the amplitude of these contracted basis functions is sufficiently small on the surface of the localization region, which is guaranteed by the optimization procedure described in this work. This approach reduces the computational costs of DFT calculations, and can be combined with sparse matrix algebra to obtain linear scaling with respect to the number of electrons in the system. Calculations on systems of 10,000 atoms or more thus become feasible in a systematic basis set with moderate computational resources. Further computational savings can be achieved by exploiting the similarity of the contracted basis functions for closely related environments, e.g. in geometry optimizations or combined calculations of neutral and charged systems.  [Show abstract] [Hide abstract]
ABSTRACT: We study within the manybody Green's function GW and BetheSalpeter formalisms the excitation energies of a paradigmatic model dipeptide, focusing on the four lowestlying local and chargetransfer excitations. Our GW calculations are performed at the selfconsistent level, updating first the quasiparticle energies, and further the singleparticle wavefunctions within the static Coulombhole plus screenedexchange approximation to the GW selfenergy operator. Important level crossings, as compared to the starting KohnSham LDA spectrum, are identified. Our final BetheSalpeter singlet excitation energies are found to agree, within 0.07 eV, with CASPT2 reference data, except for one chargetransfer state where the discrepancy can be as large as 0.5 eV. Our results agree best with LCBLYP and CAMB3LYP calculations with enhanced longrange exchange, with a 0.1 eV mean absolute error. This has been achieved employing a parameterfree formalism applicable to metallic or insulating extended or finite systems.  [Show abstract] [Hide abstract]
ABSTRACT: With the everincreasing sophistication of codes, the verification of the implementation of advanced theoretical formalisms becomes critical. In particular, cross comparison between different codes provides a strong hint in favor of the correctness of the implementations, and a measure of the (hopefully small) possible numerical differences. We lead a rigorous and careful study of the quantities that enter in the calculation of the zeropoint motion renormalization of the direct band gap of diamond due to electronphonon coupling, starting from the total energy, and going through the computation of phonon frequencies and electronphonon matrix elements. We rely on two independent implementations : Quantum Espresso + Yambo and ABINIT. We provide the order of magnitude of the numerical discrepancies between the codes, that are present for the different quantities: less than $10^{5}$ Hartree per atom on the total energy (5.722 Ha/at), less than 0.07 cm$^{1}$ on the $\Gamma,L,X$ phonon frequencies (555 to 1330 cm$^{1}$), less than 0.5% on the square of the electronphonon matrix elements and less than 4 meV on the zeropoint motion renormalization of each eigenenergies (44 to 264 meV). Within our approximations, the DFT converged direct band gap renormalization in diamond due to the electronphonon coupling is 0.409 eV (reduction of the band gap).  [Show abstract] [Hide abstract]
ABSTRACT: So far, no boron fullerenes were synthesized: more compact sp(3)bonded clusters are energetically preferred. To circumvent this, metallic clusters have been suggested by Pochet et al. [Phys. Rev. B 83, 081403(R) (2011)] as "seeds" for a possible synthesis which would topologically protect the sp(2) sector of the configuration space. In this paper, we identify a basic pentagonal unit which allows a balance between the release of strain and the selfdoping rule. We formulate a guiding principle for the stability of boron fullerenes, which takes the form of an isolated filled pentagon rule (IFPR). The role of metallic clusters is then reexamined. It is shown that the interplay of the IFPR and the seedinduced doping breaks polymorphism and its related problems: it can effectively select between different isomers and reduce the reactivity of the boron shells. The balance between self and exterior doping represents the best strategy for boron buckyball synthesis. 
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ABSTRACT: Magnetism in two dimensional atomic sheets has attracted considerable interest as its existence could allow the development of electronic and spintronic devices. The existence of magnetism is not sufficient for devices, however, as states must be addressable and modifiable through the application of an external drive. We show that defects in hexagonal boron nitride present a strong interplay between the NN distance in the edge and the magnetic moments of the defects. By stressinduced geometry modifications, we change the ground state magnetic moment of the defects. This control is made possible by the triangular shape of the defects as well as the strong spin localisation in the magnetic state.  [Show abstract] [Hide abstract]
ABSTRACT: The modifications of the electronic band structure of solids due to electronphonon interactions (temperature and zeropoint motion effects) have been explored by Manuel Cardona from both the experimental and theoretical sides. In the present contribution, we focus on the theoretical approaches to such effects. Although the situation has improved since the seventies, the wish for a fully developed theory (and associated efficient implementations) is not yet fulfilled. We review noticeable semiempirical and firstprinciple studies, with a special emphasis on the AllenHeineCardona (AHC) approach. We then focus on the nondiagonal DebyeWaller contribution, appearing beyond the rigidion approximation, in a DensityFunctional Theory (DFT) approach. A numerical study shows that they can be sizeable (10%–50%) for diatomic molecules. We also present the basic idea of a new formalism, based on DensityFunctional Perturbation Theory, that allows one to avoid the sums over a large number of empty states, and speed up the calculation by one order of magnitude, compared to the straightforward implementation of the AHC approach within DFT.  [Show abstract] [Hide abstract]
ABSTRACT: We present a firstprinciples study of Peierls distortions in transpolyacetylene, polyacene, and armchair (n,n) carbon nanotubes. Our findings suggest that the groundstate geometries of armchair (n,n) carbon nanotubes, with n up to 6, exhibit a Peierls distortion as it is found for transpolyactetylene. In contrast to previous studies in which no Peierls distortion is found with conventional local and semilocal density functionals, we use a hybrid functional whose exactexchange admixture has been specifically optimized for the problem at hand.  [Show abstract] [Hide abstract]
ABSTRACT: The energy bands of semiconductors exhibit significant shifts and broadening with temperature at constant volume. This is an effect of the direct renormalization of band energies due to electronphonon interactions. In search of an efficient linear response DFT approach to this effect, beyond semiempirical approximation or frozen phonon DFT, we have implemented formulas derived by Allen and Heine [J. Phys. C 9, 2305 (1976)] inside the ABINIT package. We have found that such formulas need a great number of bands, O(1000), to properly converge the thermal corrections of deep potential well atoms, i.e. elements of the first row. This leads to heavy computational costs even for simple systems like diamond. The DFPT formalism can be used to circumvent entirely the need for conduction bands by computing the firstorder wavefunctions using the selfconsistent Sternheimer equation. We will compare the results of both formalism demonstrating that the DFPT approach reproduces the correct converged results of the formulas of Allen and Heine.  [Show abstract] [Hide abstract]
ABSTRACT: ABINIT [http://www.abinit.org] allows one to study, from firstprinciples, systems made of electrons and nuclei (e.g. periodic solids, molecules, nanostructures, etc.), on the basis of DensityFunctional Theory (DFT) and ManyBody Perturbation Theory. Beyond the computation of the total energy, charge density and electronic structure of such systems, ABINIT also implements many dynamical, dielectric, thermodynamical, mechanical, or electronic properties, at different levels of approximation.The present paper provides an exhaustive account of the capabilities of ABINIT. It should be helpful to scientists that are not familiarized with ABINIT, as well as to already regular users. First, we give a broad overview of ABINIT, including the list of the capabilities and how to access them. Then, we present in more details the recent, advanced, developments of ABINIT, with adequate references to the underlying theory, as well as the relevant input variables, tests and, if available, ABINIT tutorials.Program summaryProgram title: ABINITCatalogue identifier: AEEU_v1_0Distribution format: tar.gzJournal reference: Comput. Phys. Comm.Programming language: Fortran95, PERL scripts, Python scriptsComputer: All systems with a Fortran95 compilerOperating system: All systems with a Fortran95 compilerHas the code been vectorized or parallelized?: Sequential, or parallel with proven speedup up to one thousand processors.RAM: Ranges from a few Mbytes to several hundred Gbytes, depending on the input file.Classification: 7.3, 7.8External routines: (all optional) BigDFT [1], ETSF IO [2], libxc [3], NetCDF [4], MPI [5], Wannier90 [6]Nature of problem: This package has the purpose of computing accurately material and nanostructure properties: electronic structure, bond lengths, bond angles, primitive cell size, cohesive energy, dielectric properties, vibrational properties, elastic properties, optical properties, magnetic properties, nonlinear couplings, electronic and vibrational lifetimes, etc.Solution method: Software application based on DensityFunctional Theory and ManyBody Perturbation Theory, pseudopotentials, with planewaves, ProjectorAugmented Waves (PAW) or wavelets as basis functions.Running time: From less than one second for the simplest tests, to several weeks. The vast majority of the >600 provided tests run in less than 30 seconds.References:[1] http://inac.cea.fr/LSim/BigDFT.[2] http://etsf.eu/index.php?page=standardization.[3] http://www.tddft.org/programs/octopus/wiki/index.php/Libxc.[4] http://www.unidata.ucar.edu/software/netcdf.[5] http://en.wikipedia.org/wiki/MessagePassingInterface.[6] http://www.wannier.org. 
Article: First principles electronic properties investigation of polythienoacene and its derivatives
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ABSTRACT: The electronic properties of laddertype polythiophene (polythienoacene) and its derivatives are studied using density functional theory. Upon an analysis of the variation of the band gap when comparing the nonladder and the laddertype polymers, a discrepancy is found between the thiophene and the pyrrole(nitrogensubstituted thiophene) polymer families. The polythienoacene has a larger band gap than the polythiophene whereas the opposite is found for the pyrrole polymers. Also, it is found that a simple alternation of the sulfur atom in polythienoacene structure by nitrogen or boron atoms can lead to small band gap polymers. The excitations of these polythienoacene's derivatives are investigated using timedependent density functional theory.
Publication Stats
992  Citations  
90.34  Total Impact Points  
Top Journals
Institutions

2015

Cea Leti
Grenoble, RhôneAlpes, France


20142015

University of Grenoble
Grenoble, RhôneAlpes, France 
French National Centre for Scientific Research
Lutetia Parisorum, ÎledeFrance, France


20132014

University Joseph Fourier  Grenoble 1
 Institut Néel
Grenoble, RhôneAlpes, France 
Institut Néel
Grenoble, RhôneAlpes, France


2010

Université du Québec à Montréal
 Department of Chemistry
Montréal, Quebec, Canada


20052009

Université de Montréal
Montréal, Quebec, Canada
