Studies of Spuriously Shifting Resonances in Time-dependent Density Functional Theory

TL;DR

This paper investigates unphysical time-dependent shifts in resonance positions in TDDFT caused by adiabatic approximations, analyzing the conditions for resonance invariance and demonstrating their impact on charge-transfer dynamics.

Contribution

It provides a detailed derivation of the exact conditions for resonance invariance in TDDFT and explores how their violation affects dynamic simulations.

Findings

Violation of the exact condition causes unphysical resonance shifts.

Adjusting the driving field frequency improves charge transfer.

Special cases show invariance of resonances under certain reference states.

Abstract

Adiabatic approximations in time-dependent density functional theory (TDDFT) will in general yield unphysical time-dependent shifts in the resonance positions of a system driven far from its ground-state. This spurious time-dependence is rationalized in [J. I. Fuks, K. Luo, E. D. Sandoval and N. T. Maitra, Phys. Rev. Lett. {\bf 114}, 183002 (2015)] in terms of the violation of an exact condition by the non-equilibrium exchange-correlation kernel of TDDFT. Here we give details on the derivation and discuss reformulations of the exact condition that apply in special cases. In its most general form, the condition states that when a system is left in an arbitrary state, in the absence of time-dependent external fields nor ionic motion, the TDDFT resonance position for a given transition is independent of the state. Special cases include the invariance of TDDFT resonances computed with respect to any reference interacting stationary state of a fixed potential, and with respect to any choice of appropriate stationary Kohn-Sham reference state. We then present several case studies, including one that utilizes the adiabatically-exact approximation, that illustrate the conditions and the impact of their violation on the accuracy of the ensuing dynamics. In particular, charge-transfer across a long-range molecule is hampered, and we show how adjusting the frequency of a driving field to match the time-dependent shift in the charge-transfer resonance frequency, results in a larger charge transfer over time.