Proton-transfer dynamics in ionized water chains using real-time Time Dependent Density Functional Theory

TL;DR

This study uses real-time TDDFT to explore how charge localization influences proton transfer in ionized water chains, revealing the impact of self-interaction errors and initial charge localization on transfer probabilities.

Contribution

It demonstrates how extended water chains reduce hemibond formation, affecting proton transfer, and compares real-time and adiabatic dynamics to clarify methodological differences.

Findings

Proton transfer probability peaks in longer water chains.

Self-interaction errors influence charge localization and transfer.

Differences between real-time and adiabatic dynamics are explained.

Abstract

In density functional-theoretic studies of photoionized water-based systems, the role of charge localization in proton-transfer dynamics is not well understood. This is due to the inherent complexity in extracting the contributions of coupled electron-nuclear non-adiabatic dynamics in the presence of exchange and correlation functional errors. In this work, we address this problem by simulating a model system of ionized linear H-bonded water clusters using real-time Time Dependent Density Functional Theory (rt-TDDFT)-based Ehrenfest dynamics. Our aim is to understand how self-interaction error in semilocal exchange and correlation functionals affects the probability of proton transfer. In particular, we show that for H-bonded (H$_2$O)$_n^+$ chains (with $n>3$), the proton-transfer probability attains a maximum, becoming comparable to that predicted by hybrid functionals. This is because the formation of hemibonded-type geometries is largely suppressed in extended H-bonded structures. We also show how the degree of localization of the initial photo-hole is connected to the probability of a proton-transfer reaction, as well as to the separation between electronic and nuclear charge. These results are compared to those obtained with adiabatic dynamics where the initial wavefunction is allowed to relax to the ground state of the ion cluster, explaining why different functionals and dynamical approaches lead to quantitatively different results.