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 cHF and show that there exists one value of cHF (∼0.32) that reproduces at least semi-quantitatively the optical gap of this material.
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7 June 2015
Research Article|
June 03 2015
Optical properties of alkali halide crystals from all-electron hybrid TD-DFT calculations Available to Purchase
R. Webster;
R. Webster
a)
1Thomas Young Centre, Department of Chemistry,
Imperial College London
, South Kensington, London SW7 2AZ, United Kingdom
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L. Bernasconi;
L. Bernasconi
2Rutherford Appleton Laboratory,
STFC
, Harwell Oxford, Didcot OX11 0QX, United Kingdom
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N. M. Harrison
N. M. Harrison
1Thomas Young Centre, Department of Chemistry,
Imperial College London
, South Kensington, London SW7 2AZ, United Kingdom
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1Thomas Young Centre, Department of Chemistry,
Imperial College London
, South Kensington, London SW7 2AZ, United Kingdom
2Rutherford Appleton Laboratory,
STFC
, Harwell Oxford, Didcot OX11 0QX, United Kingdom
a)
Author to whom correspondence should be addressed. Electronic mail: [email protected]
J. Chem. Phys. 142, 214705 (2015)
Article history
Received:
December 01 2014
Accepted:
May 18 2015
Citation
R. Webster, L. Bernasconi, N. M. Harrison; Optical properties of alkali halide crystals from all-electron hybrid TD-DFT calculations. J. Chem. Phys. 7 June 2015; 142 (21): 214705. https://doi.org/10.1063/1.4921822
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