Diabatic states of charge transfer with constrained charge equilibration

TL;DR

This paper introduces CQEq, a simple and computationally affordable method to obtain diabatic states and energies for charge transfer processes, matching more complex methods like CDFT.

Contribution

The paper presents CQEq, a new constrained charge equilibration approach for calculating diabatic states, offering an efficient alternative to constrained DFT for complex systems.

Findings

CQEq accurately predicts diabatic energies and charges.

CQEq provides adiabatic excitation energies in good agreement with CDFT.

Method is promising for use with machine learning force fields.

Abstract

Charge transfer (CT) processes that are electronically non-adiabatic are ubiquitous in chemistry, biology, and materials science, but their theoretical description requires diabatic states or adiabatic excited states. For complex systems, these latter states are more difficult to calculate than the adiabatic ground state. Here, we propose a simple method to obtain diabatic states, including energies and charges, by constraining the atomic charges within the charge equilibration framework. For two-state systems, the exact diabatic coupling can be determined, from which the adiabatic excited-state energy can also be calculated. The method can be viewed as an affordable alternative to constrained density functional theory (CDFT), and so we call it constrained charge equilibration (CQEq). We test the CQEq method on the anthracene-tetracyanoethylene CT complex and the reductive decomposition of ethylene carbonate on a lithium metal surface. We find that CQEq predicts diabatic energies, charges, and adiabatic excitation energies in good agreement with CDFT, and we propose that CQEq is promising for combination with machine learning force fields to study non-adiabatic CT in the condensed phase.