Long-range excitations in time-dependent density functional theory

TL;DR

This paper addresses the failure of adiabatic time-dependent density functional theory in describing long-range excitations in heteroatomic molecules, proposing a new non-empirical exchange-correlation kernel to improve accuracy at large separations.

Contribution

The authors derive an approximate non-empirical kernel that captures static correlation effects in large-separation molecular dissociation, improving TDDFT predictions.

Findings

The new kernel accounts for static correlation in dissociation.

It corrects the failure of adiabatic TDDFT for charge-transfer excitations.

Results are relevant for local and semi-local ground-state approximations.

Abstract

Adiabatic time-dependent density functional theory fails for excitations of a heteroatomic molecule composed of two open-shell fragments at large separation. Strong frequency-dependence of the exchange-correlation kernel is necessary for both local and charge-transfer excitations. The root of this is static correlation created by the step in the exact Kohn-Sham ground-state potential between the two fragments. An approximate non-empirical kernel is derived for excited molecular dissociation curves at large separation. Our result is also relevant for the usual local and semi-local approximations for the ground-state potential, as static correlation there arises from the coalescence of the highest occupied and lowest unoccupied orbital energies as the molecule dissociates.