Optical properties of alkali halide crystals from all-electron hybrid TD-DFT calculations

Journal Article · Sun Jun 07 00:00:00 EDT 2015 · Journal of Chemical Physics

DOI:https://doi.org/10.1063/1.4921822· OSTI ID:22415949

Harrison, N. M. ^[1]; Bernasconi, L. ^[2]

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1. Thomas Young Centre, Department of Chemistry, Imperial College London, South Kensington, London SW7 2AZ (United Kingdom)
2. Rutherford Appleton Laboratory, STFC, Harwell Oxford, Didcot OX11 0QX (United Kingdom)

We present a study of the electronic and optical properties of a series of alkali halide crystals AX, with A = Li, Na, K, Rb and X = F, Cl, Br based on a recent implementation of hybrid-exchange time-dependent density functional theory (TD-DFT) (TD-B3LYP) in the all-electron Gaussian basis set code CRYSTAL. We examine, in particular, the impact of basis set size and quality on the prediction of the optical gap and exciton binding energy. The formation of bound excitons by photoexcitation is observed in all the studied systems and this is shown to be correlated to specific features of the Hartree-Fock exchange component of the TD-DFT response kernel. All computed optical gaps and exciton binding energies are however markedly below estimated experimental and, where available, 2-particle Green’s function (GW-Bethe-Salpeter equation, GW-BSE) values. We attribute this reduced exciton binding to the incorrect asymptotics of the B3LYP exchange correlation ground state functional and of the TD-B3LYP response kernel, which lead to a large underestimation of the Coulomb interaction between the excited electron and hole wavefunctions. Considering LiF as an example, we correlate the asymptotic behaviour of the TD-B3LYP kernel to the fraction of Fock exchange admixed in the ground state functional c{sub HF} and show that there exists one value of c{sub HF} (∼0.32) that reproduces at least semi-quantitatively the optical gap of this material.

• OSTI ID: 22415949
• Journal Information: Journal of Chemical Physics, Vol. 142, Issue 21; Other Information: (c) 2015 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA); ISSN 0021-9606
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
ASYMPTOTIC SOLUTIONS
BETHE-SALPETER EQUATION
BINDING ENERGY
CORRELATIONS
CRYSTALS
DENSITY FUNCTIONAL METHOD
ELECTRONS
EXCITONS
GROUND STATES
HARTREE-FOCK METHOD
HOLES
KERNELS
LITHIUM FLUORIDES
OPTICAL PROPERTIES
PARTICLES
RUBIDIUM BROMIDES
TIME DEPENDENCE
WAVE FUNCTIONS