Understanding the interplay between hole localization and reactivity in photoionized water clusters using real-time Time-Dependent Density Functional Theory

Photocatalytic water-splitting on semiconductor surfaces is of topical interest for renewable energy applications, and yet, the molecular intermediates and their mutual interactions underlying photocatalysis remain poorly understood. In this work, we examine the scope of rt-TDDFT-based methods for analyzing ultrafast processes in photoexcited systems. We study the response of a prototypical molecular system consisting of chains of H-bonded (H₂O)_n molecules (n=2-5) under photoionization. The time evolution of the photogenerated hole is captured by rt-TDDFT Ehrenfest dynamics. We use a generalized gradient approximation (GGA-PBE) for nonadiabatic electron-ion dynamics, justified by comparing dynamic hole densities computed at PBE with those at higher PBE0 level of the theory. We also compare results from rt-TDDFT dynamics with those from adiabatic Born-Oppenheimer molecular dynamics at the GGA level, elucidating the importance of incorporating explicit nonadiabatic effects in excited-state phenomena. The H-bond cooperativity effects in (H₂O)_n⁺ chains are identified, emphasizing their role in facilitating hole localization. Finally, we also uncover new connections between the test system and the H-bond network formed at water-semiconductor interfaces.