Proton-transfer dynamics in ionized water chains using real-time time-dependent density functional theory

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 nonadiabatic 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-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 the proton-transfer probability is largely underestimated for short H-bonded chains but becomes comparable to that predicted by hybrid functionals for (H2O)n+ chains with n>3. 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 hole-proton separation. These results are compared to those obtained with adiabatic dynamics where the initial wave function is allowed to relax to the ground state of the ion cluster, explaining why different functionals and dynamical approaches lead to quantitatively different results.