Publications (37)86.51 Total impact
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ABSTRACT: We propose a new approach to identifying and rationalizing the contribution of core electron polarization to dielectric screening, based on ab initio calculations of the dielectric matrix in its eigenpotential basis. We also present calculations of phonon frequencies, dielectric constants, and quasiparticle energies of several systems, and we discuss the quantitative effect of including core polarization. Our findings illustrate efficient ways of approximating the spectral decomposition of dielectric matrices used, e.g., in manybody perturbation theory and dielectric constant calculations, with substantial computational gains for large systems composed of heavy atoms.  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the influence of core polarization on the dielectric screening of atoms, molecules and solids with focus on non metallic systems. We compare results for dielectric band structures and for the eigenvalues of the dielectric matrix obtained by varying the number of valence electrons included in our calculations. We show that (semi)core electronic states may substantially influence the dielectric screening, even if they lie very deep in energy compared to the outermost valence electrons. We then discuss how the changes in dielectric screening observed when including (semi)core electrons affect computed quasi particle energies at the GW level, and phonon frequencies, e.g. the LOTO splitting. We focus on closed shell atoms, including Be,Mg, Ca, Ar, Zn, simple diatomic molecules and simple ionic solids, e.g. LiH and NaH.  [Show abstract] [Hide abstract]
ABSTRACT: A method to solve the BetheSalpeter equation that avoids the explicit calculation of empty electronic states and the storage and inversion of dielectric matrices has been recently introduced [13]. This approach is suitable to compute the absorption spectra of large systems in a wide energy range and without relying on the TammDancoff approximation. We show the accuracy and scalability of this method by presenting calculations of absorption spectra of solids, molecules and nanostructures, including Si quantum dots and nanowires. In the case of nanowires, we discuss the influence of size and surface reconstruction on the optical properties.[4pt] [1] D. Rocca, D. Lu, and G. Galli, J. Chem. Phys. 133, 164109 (2010)[0pt] [2] D. Rocca, Y. Ping, R. Gebauer, and G. Galli, submitted to PRB [0pt] [3] Y. Ping, D. Rocca, D. Lu, and G. Galli, submitted to PRB  [Show abstract] [Hide abstract]
ABSTRACT: We present a study of the optical absorption spectra of thin silicon nanowires using manybody perturbation theory. We solve the BetheSalpeter equation in the static approximation using a technique that avoids explicit calculation of empty electronic states, as well as storage and inversion of the dielectric matrix. We provide a detailed assessment of the numerical accuracy of this technique, when using plane wave basis sets and periodically repeated supercells. Our calculations show that establishing numerical error bars of computed spectra is critical, in order to draw meaningful comparisons with experiments and between results obtained within different algorithms. We also discuss the influence of surface structure on the absorption spectra of nanowires with ≃1nm diameter. Finally, we compare our calculations with those obtained within timedependent density functional theory and find substantial differences, more pronounced than in the case of Si nanoparticles with the same diameter.  [Show abstract] [Hide abstract]
ABSTRACT: The description of van der Waals dispersion interactions using the so called EXX/RPA method has recently attracted a widespread interest. Overall, equilibrium distances and cohesive energies of weakly bound molecular systems exhibit a significant improvement over the the results of semilocal Density Functional Theory calculations [1,2], due to the proper inclusion of longrange correlation effects. However, cohesive energies still result to be underestimated with respect to experiments in several cases. This is mainly due to the neglect of the exchangecorrelation kernel in evaluating response functions entering the correlation energy expression. In this work, we study the effect of several model exchangecorrelation kernels and evaluate their performance for molecular systems. [4pt] [1] D. Lu, Y. Li, D. Rocca and G. Galli, Phys. Rev. Lett. 102, 206411 (2009)[0pt] [2] Y. Li, D. Lu, HV Nguyen and G. Galli, J. Phys. Chem. A, 114, 19441952 (2010) and D. Lu, HV Nguyen, and G. Galli, J. Chem. Phys. 133, 154110 (2010) 
Article: Abinitio calculations of absorption spectra of nanowires by solving the BetheSalpeter Equation
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ABSTRACT: A first principle approach to the solution of the Bethe Salpeter equation without empty electronic states has been recently developed [1], which makes possible the calculations of absorption spectra of relatively large systems (with several hundreds of electrons). We present applications of this approach to quasione dimensional systems, including chains of hydrogen molecules and Si nanowires. We discuss techniques to further improve the performance of absorption spectra calculations, and present a general scheme to accurately integrate the divergence in the screened exchange integrals. Finally, in the case of Si nanowires, we discuss the effect of surface reconstruction in shaping optical absorption spectra.[4pt] [1] D. Rocca, D. Lu and G. Galli, J. Chem. Phys. 133, 164109 (2010)  [Show abstract] [Hide abstract]
ABSTRACT: Lithium is known to exist in different phases and to uperconduct under pressure (P). We investigate the screened electronelectron interaction in Li as a function of P by analyzing the dielectric band structures of several phases. These band structures are obtained by iterative diagonalization ootnotetextH.Wilson, F.Gygi and G.Galli, Phys. Rev. B, 78,113303 (2008); Hugh F. Wilson, Deyu Lu, Francois Gygi Phys. Rev. B. 79, 245106 (2009). of the dielectric matrix as a function of wave vector and frequency. Even though the superconductivity in lithium is electronphonon mediated, lithium is a good test system to study the screened ee interaction more generally, which might be a primary mechanism for the superconductivity in high Tc nitrides of the form MNCl (M=Ti,Zr,Hf).  [Show abstract] [Hide abstract]
ABSTRACT: We describe an ab initio approach to compute the optical absorption spectra of molecules and solids, which is suitable for the study of large systems and gives access to spectra within a wide energy range. In this approach, the quantum Liouville equation is solved iteratively within first order perturbation theory, with a Hamiltonian containing a static selfenergy operator. This procedure is equivalent to solving the statically screened BetheSalpeter equation. Explicit calculations of single particle excited states and inversion of dielectric matrices are avoided using techniques based on density functional perturbation theory. In this way, full absorption spectra may be obtained with a computational workload comparable to ground state HartreeFock calculations. We present results for small molecules, for the spectra of a 1 nm Si cluster in a wide energy range (20 eV), and for a dipeptide exhibiting charge transfer excitations.  [Show abstract] [Hide abstract]
ABSTRACT: We derive a power expansion of the correlation energy of weakly bound systems within the random phase approximation (RPA), in terms of the Coulomb interaction operator, and we show that the asymptotic limit of the second and thirdorder terms yields the van der Waals (vdW) dispersion energy terms derived by ZarembaKohn and AxilrodTeller within perturbation theory. We then show that the use of the secondorder expansion of the RPA correlation energy results in rather inaccurate binding energy curves for weakly bonded systems, and discuss the implications of our findings for the development of approximate vdW density functionals. We also assess the accuracy of different exchange energy functionals used in the derivation of vdW density functionals.  [Show abstract] [Hide abstract]
ABSTRACT: A first principle approach to solve the Bethe Salpeter equation has been recently proposed [1], that does not require the calculation of excited single particle orbitals, and thus opens the way to calculations of absorption spectra of relatively large systems. We show that the efficiency of this approach can be further improved: (i) by exploiting localization properties of the eigenvectors of the dielectric matrix entering the expression of the screened Coulomb interaction; and (ii) by adopting appropriate truncation schemes that allows one to accurately describe the dielectric matrix [2] with a small number of eigenvectors and eigenvalues . Applications to the calculation of absorption spectra of semiconducting clusters and Si nanorods will be presented. [1]D.Rocca, D.Lu and G.galli (submitted) [2]H.Wilson, F.Gygi and G.Galli, Phys. Rev. B 78, 113303 (2008); and H. Wilson, D. Lu, F. Gygi and G. Galli, Phys. Rev. B, 79, 245106, 2009.  [Show abstract] [Hide abstract]
ABSTRACT: A recently developed technique to diagonalize iteratively dielectric matrices [1], is used to carry out efficient, abinitio calculations of dispersion interactions, and excited state properties of nanostructures. In particular, we present results for the binding energies of weakly bonded molecular crystals [2], obtained at the EXX/RPA level of theory, and for absorption spectra of semiconducting clusters, obtained by an iterative solution of the BetheSalpeter equations [3]. We show that the ability to obtain the eigenmodes of dielectric matrices from Density Functional perturbation theory, without computing single particle excited states, greatly improves the efficiency of both EXX/RPA and many body perturbation theory [3,4] calculations and opens the way to large scale computations. [1] H. Wilson, F. Gygi and G. Galli, Phys. Rev. B , 78, 113303, 2008; and H. Wilson, D. Lu, F. Gygi and G. Galli, Phys. Rev. B, 79, 245106, 2009. [2] D. Lu, Y. Li, D. Rocca and G. Galli, Phys. Rev. Lett, 102, 206411, 2009; and Y. Li, D. Lu, V. Nguyen and G. Galli, J. Phys. Chem. C (submitted) [3] D. Rocca, D. Lu and G. Galli, submitted. [4] D. Lu, F. Gygi and G. Galli, Phys. Rev. Lett. 100, 147601, 2008. Work was funded by DOE/Scidac DEFC0206ER25794 and DOE/BES DEFG0206ER46262. 
Article: BetheSalpeter equation without empty electronic states applied to chargetransfer excitations
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ABSTRACT: We present an approach to compute optical absorption spectra of molecules and nanostructures from first principles, which is suitable for the study of large systems and gives access to spectra within a wide energy range. In this approach, the quantum Liouville equation is solved iteratively within first order perturbation theory, with a Hamiltonian containing a static selfenergy operator. This is equivalent to solving the BetheSalpeter equation. Explicit calculations of single particle excited states and inversion of dielectric matrices are avoided using techniques based on Density Functional Perturbation Theory [1,2]. In this way, full absorption spectra may be obtained with a computational workload comparable to ground state HartreeFock calculations. Applications to the description of charge transfer excitations are presented. [1] D.Rocca, D.Lu and G.Galli (submitted) [2] H. Wilson, F. Gygi and G. Galli, Phys. Rev. B , 78, 113303, 2008;H. Wilson, D. Lu, F. Gygi and G. Galli, Phys. Rev. B, 79, 245106,2009.  [Show abstract] [Hide abstract]
ABSTRACT: We have investigated intermolecular interactions in weakly bonded molecular assemblies from first principles, by combining exact exchange energies (EXX) with correlation energies defined by the adiabatic connection fluctuationdissipation theorem, within the random phase approximation (RPA)[1,2]. We present results for three different types of molecular systems: the benzene crystal, the methane crystal and selfassembled monolayers of phenylenediisocyanide. We describe in detail how computed equilibrium lattice constants and cohesive energies may be affected by input ground state wave functions and orbital energies, by the geometries of the molecular monomers in the assemblies, and by the inclusion of zero point energy contribution to the total energy. We find that the EXX/RPA perturbative approach provides an overall satisfactory, first principle description of dispersion forces, in good agreement with experiments and advanced quantum chemistry results. However, binding energies tend to be underestimated and possible reasons for this discrepancy are discussed. This work was funded by DOE/BES DEFG0206ER46262 and DOE/SciDAC DEFC0206ER25794.[1] Y. Li, D. Lu, HV. Nguyen and G. Galli, J. Phys. Chem.(submitted). [2]D. Lu, Y. Li, D. Rocca and G. Galli, Phys. Rev. Lett. 102, 206411(2009).  [Show abstract] [Hide abstract]
ABSTRACT: We investigated intermolecular interactions in weakly bonded molecular assemblies from first principles, by combining exact exchange energies (EXX) with correlation energies defined by the adiabatic connection fluctuationdissipation theorem, within the random phase approximation (RPA). We considered three different types of molecular systems: the benzene crystal, the methane crystal, and selfassembled monolayers of phenylenediisocyanide, which involve aromatic rings, sp(3)hybridized CH bonds, and isocyanide triple bonds, respectively. We describe in detail how computed equilibrium lattice constants and cohesive energies may be affected by the input ground state wave functions and orbital energies, by the geometries of molecular monomers in the assemblies, and by the inclusion of zeropoint energy contribution to the total energy. We find that the EXX/RPA perturbative approach provides an overall satisfactory, firstprinciples description of dispersion forces. However, binding energies tend to be underestimated, and possible reasons for this discrepancy are discussed.  [Show abstract] [Hide abstract]
ABSTRACT: Intermolecular interactions in the van der Waals bonded benzene crystal are studied from first principles, by combining exact exchange energies with correlation energies defined by the adiabatic connection fluctuationdissipation theorem, within the random phase approximation. Correlation energies are evaluated using an iterative procedure to compute the eigenvalues of dielectric matrices, which eliminates the computation of unoccupied electronic states. Our results for the structural and binding properties of solid benzene are in very good agreement with experimental results and show that the framework adopted here is a very promising one to investigate molecular crystals and other condensed systems bound by dispersion forces.  [Show abstract] [Hide abstract]
ABSTRACT: The eigenvalues and eigenvectors of the dielectric matrix ϵ provide a compact representation of the screening properties of interacting electronic systems. We have previously shown that the dielectric eigenvalue spectrum may be efficiently computed by iterative linearresponse calculations and that for nonmetallic systems ϵ may be obtained through an eigenvalueeigenvector decomposition where only a small number of eigenvalues are included. Here we investigate the spectral properties of the dielectric matrices of a variety of systems (solids, nanostructures, and molecules) as well as the convergence properties of the eigenvalue decomposition of ϵ as a function of the number of eigenmodes. Our results provide guidance on how to perform practical calculations of dielectric matrices using iterative techniques.  [Show abstract] [Hide abstract]
ABSTRACT: We present manybody perturbation theory calculations of the electronic properties of phenylene diisocyanide selfassembled monolayers (SAMs) on a gold surface. Using structural models obtained within density functional theory (DFT), we have investigated how the SAM molecular energies are modified by selfenergy corrections and how they are affected by the presence of the surface. We have employed a combination of GW (G = Green’s function; W = screened Coulomb interaction) calculations of the SAM quasiparticle energies and a semiclassical image potential model to account for surface polarization effects. We find that it is essential to include both quasiparticle corrections and surface screening in order to provide a reasonable estimate of the energy level alignment at a SAM−metal interface. In particular, our results show that within the GW approximation the energy distance between phenylene diisocyanide SAM energy levels and the gold surface Fermi level is much larger than that found within DFT, e.g., more than double in the case of low packing densities of the SAM.  [Show abstract] [Hide abstract]
ABSTRACT: We present a novel first principle approach to solve the BetheSalpeter equation (BSE) that builds on recent progress in timedependent density functional perturbation theory [1], and uses an eigenvalue decomposition representation of the dielectric matrix [2]. This approach does not require the explicit calculation of excited single particle electronic states, making it suitable for calculations involving large basis sets and/or a large number of transitions. The numerical solution of the BSE is obtained through a generalized, nonHermitian Lanczos iterative algorithm and does not require the use of the TammDancoff approximation. Furthermore, since Lanczos coefficients are frequency independent, optical spectra may be obtained in a very broad energy range. The efficiency and accuracy of the new approach are demonstrated by calculating the optical properties of silicon nanoclusters with up to 1 nm diameter. [1] D. Rocca, R. Gebauer, Y Saad, and S. Baroni, J. Chem. Phys. 128, 154105 (2008). [2] H.Wilson, F.Gygi and G.Galli, Phys. Rev. B 78, 113303 (2008). 
Article: First principle calculations of long range correlation effects within the random phase approximation
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ABSTRACT: The local and semilocal approximations to Density Functional Theory fail to describe correctly certain types of weak interactions (e.g. van der Waals forces) due an incorrect account of long range correlation effects. Such effects may be described by computing correlation energies within the random phase approximation (RPA), using the fluctuationdissipation theorem and the adiabatic connection. We present an approach to compute RPA correlation energies based on an eigenmode expansion of the dielectric matrix [1,2]. By solving the frequency dependent Sternheimer equation within linear response theory [3], we eliminate the need to compute single particle unoccupied states, which makes our approach more efficient than methods using the directsummation technique. Furthermore, the use of a dielectric eigenmode representation allows for a physical interpretation of several, different contributions to correlation energies. Results for graphite and the benzene crystal will be discussed. [1] H. Wilson, F. Gygi and G. Galli, Phys. Rev. B, 78:113303, (2008). [2] D. Lu, F. Gygi and G. Galli, Phys. Rev. Lett., 100:147601(2008). [3] S. Baroni, S. de Gironcoli, A. Dal Corso, and P. Giannozzi, Rev. Mod. Phys. 73:515, (2001).  [Show abstract] [Hide abstract]
ABSTRACT: We present a firstprinciples study of the static dielectric properties of ice and liquid water. The eigenmodes of the dielectric matrix E are analyzed in terms of maximally localized dielectric functions similar, in their definition, to maximally localized Wannier orbitals obtained from Bloch eigenstates of the electronic Hamiltonian. We show that the lowest eigenmodes of E (1) are localized in real space and can be separated into groups related to the screening of lone pairs, intra, and intermolecular bonds, respectively. The local properties of the dielectric matrix can be conveniently exploited to build approximate dielectric matrices for efficient, yet accurate calculations of quasiparticle energies.
Publication Stats
697  Citations  
86.51  Total Impact Points  
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Institutions

2012

Brookhaven National Laboratory
Брукхейвен, New York, United States


20082010

University of California, Davis
 Department of Chemistry
Davis, California, United States


20022006

University of Illinois, UrbanaChampaign
 • Beckman Institute for Advanced Science and Technology
 • Department of Physics
Urbana, Illinois, United States
