Electron charge dynamics and charge separation: A response theory approach

TL;DR

This paper uses response theory to analyze electron charge dynamics and charge separation, comparing linear and quadratic responses, and proposing approximations to better describe charge transfer processes.

Contribution

It demonstrates that quadratic response theory effectively captures charge dynamics and separation, offering improvements over linear response in modeling charge transfer systems.

Findings

Quadratic response closely matches exact time propagation results in certain regimes.

Linear response accurately describes optical properties but not charge separation.

Proposed approximations improve the accuracy of quadratic response modeling.

Abstract

This study applies response theory to investigate electron charge dynamics, with a particular focus on charge separation. We analytically assess the strengths and limitations of linear and quadratic response theories in describing charge density and current, illustrated by a model that simulates charge transfer systems. While linear response accurately captures optical properties, the quadratic response contains the minimal ingredients required to describe charge dynamics and separation. Notably, it closely matches exact time propagation results in some regime that we identify. We propose and test several approximations to the quadratic response and explore the influence of higher-order terms and the effect of an on-site interaction $U$.