Electronic excitations from a linear-response range-separated hybrid scheme

TL;DR

This paper evaluates a linear-response range-separated hybrid DFT scheme for calculating electronic excitation energies, showing it improves charge-transfer and Rydberg state predictions but has limitations for low-lying triplet states.

Contribution

It introduces and assesses a linear-response RSH scheme combining long-range HF exchange with short-range DFT, offering an alternative to existing long-range correction methods.

Findings

Improves charge-transfer and Rydberg excitation energy predictions.

Underestimates low-lying triplet state energies, mitigated by Tamm-Dancoff approximation.

Suggests RSH scheme as a good starting point for excitation energy calculations.

Abstract

We study linear-response time-dependent density-functional theory (DFT) based on the single-determinant range-separated hybrid (RSH) scheme, i.e. combining a long-range Hartree-Fock exchange kernel with a short-range DFT exchange-correlation kernel, for calculating electronic excitation energies of molecular systems. It is an alternative to the long-range correction (LC) scheme which has a standard full-range DFT correlation kernel instead of only a short-range one. We discuss the local-density approximation (LDA) to the short-range exchange-correlation kernel, and assess the performance of the linear-response RSH scheme for singlet-singlet and singlet-triplet valence and Rydberg excitations in the N2, CO, H2CO, C2H4, and C6H6 molecules, and for the first charge-transfer excitation in the C2H4-C2F4 dimer. The introduction of long-range HF exchange corrects the underestimation of charge-transfer and high-lying Rydberg excitation energies obtained with standard (semi)local density-functional approximations, but also leads to underestimated excitation energies to low-lying spin-triplet valence states which can be cured by the Tamm-Dancoff approximation. This work thus suggests that the present linear-response RSH scheme is a reasonable starting approximation for describing electronic excitation energies, even before adding an explicit treatment of long-range correlation.