Showing papers by "Artur F. Izmaylov published in 2008"

Accurate solid-state band gaps via screened hybrid electronic structure calculations

Abstract: The band energy differences of solids calculated with screened hybrid density functionals, such as the functional of Heyd-Scuseria-Ernzerhof (HSE), reproduce experimental band gaps with a high degree of accuracy. This unexpected result is here rationalized by observing that band energy differences obtained from generalized Kohn-Sham calculations with screened (short-range) Hartree-Fock-type exchange approach the excitation energies obtained via time-dependent density functional calculations with the corresponding unscreened functional. The latter are expected to be the accurate predictions of the experimental optical absorption spectra. While the optimum screening parameter (omega) is system dependent, the HSE standard value of omega=0.11 bohr(-1) represents a reasonable compromise across diverse systems.

Assessment of a Middle-Range Hybrid Functional

Abstract: While hybrid functionals are largely responsible for the utility of modern Kohn-Sham density functional theory, they are not without their weaknesses. In the solid state, the slow decay of their nonlocal Hartree-Fock-type exchange makes hybrids computationally demanding and can introduce unphysical effects. Both problems can be remedied by a screened hybrid which uses exact exchange only at short-range. Many molecular properties, in contrast, benefit from the inclusion of long-range exact exchange. Recently, the authors reconciled these two seemingly contradictory requirements by introducing the HISS functional [ J. Chem. Phys. 2007 , 127 , 221103 ], which uses exact exchange only in the middle range. In this paper, we expand upon our previous work, benchmarking the performance of the HISS functional for several simple properties and applying it to the dissociation of homonuclear diatomic cations and to the polarizability of linear H2 chains to determine the importance of middle-range exact exchange for these systems, which are expected to be sensitive to the asymptotic exchange potential.

Why are time-dependent density functional theory excitations in solids equal to band structure energy gaps for semilocal functionals, and how does nonlocal Hartree-Fock-type exchange introduce excitonic effects?

Abstract: We examine the time-dependent density functional theory (TD-DFT) equations for calculating excitation energies in solids with Gaussian orbitals and analytically show that for semilocal functionals, their lowest eigenvalue collapses to the minimum band orbital energy difference. With the introduction of nonlocal Hartree–Fock-type exchange (as in hybrid functionals), this result is no longer valid, and the lowest TD-DFT eigenvalue reflects the appearance of excitonic effects. Previously reported “charge-transfer” problems with semilocal TD-DFT excitations in molecules can be deduced from our analysis by taking the limit to infinite lattice constant.

Resolution of the identity atomic orbital Laplace transformed second order Møller-Plesset theory for nonconducting periodic systems.

Abstract: An improvement in performance of the atomic orbital Laplace transformed second-order Moller–Plesset (AO-LT-MP2) method for periodic systems is reported using the resolution of identity (RI) technique. Transformation of the two-electron integrals constitutes the main computational bottleneck of the AO-LT-MP2 method. A substitution of regular four-center integrals by their three center counterparts in the RI approximation naturally reduces the computational cost of the integral transformation step. The RI divergence problem in the presence of periodic boundary conditions is solved in our implementation by restricting the fitting domain. Accuracy and computational efficiency of the RI-AO-LT-MP2 approach are assessed on a set of one-dimensional test systems: trans-polyacetylene and anti-transoid polymethineimine.

Analytical infrared intensities for periodic systems with local basis sets

Abstract: We report a method for the efficient evaluation of analytic infrared intensities within a generalized Kohn--Sham density functional theory using Gaussian orbitals and periodic boundary conditions. A discretized form of the Berry phase is used to evaluate a periodic dipole moment and its derivatives with respect to in-phase nuclear coordinate displacements. Benchmark calculations are presented for one-dimensional chains of water molecules and poly($p$-phenylene vinylene).

The role of middle-range Hartree-Fock-type exchange in hybrid functionals

Abstract: Submitted for the MAR08 Meeting of The American Physical Society The role of middle-range Hartree-Fock-type exchange in hybrid functionals THOMAS HENDERSON, ARTUR IZMAYLOV, GUSTAVO SCUSERIA, Rice University, ANDREAS SAVIN, Universite Pierre et Marie Curie — While hybrid functionals are responsible for many successes in modern Kohn-Sham theory, they have several drawbacks. The slow decay of nonlocal exchange makes hybrids computationally demanding in extended systems with small bandgaps, while in finite systems the rapid decay of semilocal exchange causes errors in quantities sensitive to the long-range potential. Both problems can be addressed by range-separated hybrids which include nonlocal exchange only for some values of the interelectronic separation. Excluding long-range nonlocal exchange in extended systems improves computational efficiency without loss of accuracy, while including full long-range nonlocal exchange in finite systems improves accuracy without loss of efficiency. Both approaches use a significant fraction of nonlocal exchange for intermediate electronic separations. We show that a hybrid functional that uses nonlocal exchange only for this middle range has many advantages, properly describing thermochemistry, reaction barriers, and bandgaps in the same framework. Thomas Henderson Rice University Date submitted: 29 Nov 2007 Electronic form version 1.4