Recommendation of interstitial hydrogen positions in metal oxides

Hydrogen impurities in metal oxides are targets of experimental and theoretical investigations because of their crucial influences on material properties. However, systematic modeling of the hydrogen impurities is not straightforward as they show a large variety of configurations depending on the host oxides, especially at interstitial sites. In this study, we utilize the electrostatic potential, electron localization function, and charge density for recommending interstitial hydrogen positions in metal oxides. We consider (i) local minima of the electrostatic potential and (ii) local maxima of the electron localization function as interstitial proton sites, and (iii) local minima of the charge density as interstitial hydrogen-atom and hydride-ion sites. We assess the validity of these recommendations using first-principles calculations for hydrogen impurities in 20 common metal oxides and show that the combination of definitions (i)-(iii) is capable of efficiently finding the most stable interstitial configurations, especially for protons and hydride ions. The present approach can substantially narrow the search space for interstitial hydrogen positions, thereby reducing the overall computational costs for modeling hydrogen impurities.