Charge-Transfer in Time-Dependent Density Functional Theory

TL;DR

This paper reviews the challenges and recent advances in using time-dependent density functional theory (TDDFT) to accurately model charge transfer processes, highlighting issues with current functionals and the need for better dynamical features.

Contribution

It identifies key differences in exchange-correlation kernels for charge transfer and discusses the limitations of current functionals in dynamic charge transfer modeling.

Findings

Standard functionals perform poorly for long-range charge transfer.

Dynamical step and peak features are missing in current functionals.

Current functionals underestimate charge transfer and cause resonance shifts.

Abstract

Charge transfer plays a crucial role in many processes of interest in physics, chemistry, and bio-chemistry. In many applications the size of the systems involved calls for time-dependent density functional theory (TDDFT) to be used in their computational modeling, due to its unprecedented balance between accuracy and efficiency. However, although exact in principle, in practise approximations must be made for the exchange-correlation functional in this theory, and the standard functional approximations perform poorly for excitations which have a long-range charge-transfer component. Intense progress has been made in developing more sophisticated functionals for this problem, which we review. We point out an essential difference between the properties of the exchange-correlation kernel needed for an accurate description of charge-transfer between open-shell fragments and between closed-shell fragments. We then turn to charge-transfer dynamics, which, in contrast to the excitation problem, is a highly non-equilibrium, non-perturbative, process involving a transfer of one full electron in space. This turns out to be a much more challenging problem for TDDFT functionals. We describe dynamical step and peak features in the exact functional evolving over time, that are missing in the functionals currently used. The latter underestimate the amount of charge transferred and manifest a spurious shift in the charge transfer resonance position. We discuss some explicit examples.