Diffusion Monte Carlo Study of the Structure and Spectroscopy of H₃O

A potential energy surface for H₃O^- has been developed based on the NN+(MOB-ML) approach we developed for studies of complexes of OH^- with two or three water molecules. Unlike those systems, H₃O^- has two low-energy isomers, H^-·H₂O and OH^-·H₂, which differ in energy by less than 2.5 kcal mol^-1, and which are separated by a barrier of roughly 4.5 kcal mol^-1. We find that by training the NN+(MOB-ML) model using structures based on diffusion Monte Carlo (DMC) simulations initiated in the two potential minima, we are able to obtain a potential surface that describes both isomers. Using these potentials, the structure and spectra of H₃O^- and its deuterated analogues are investigated using DMC. These calculations show that the ground state wave function for H₃O^- is mainly localized in the H^-·H₂O minimum in the potential, with a small amount of the probability amplitude (<5%) in the region of the OH^-·H₂ minimum. The delocalization of the wave function into the secondary minimum is lowered by deuteration of the water molecule, while replacing H^- with D^- increases the isomerization due to the shortening of the average distance between the hydride ion and the hydrogen atom in water to which it is bound. Introducing one quantum of excitation in the H^-···H₂O stretching vibration increases the amount of isomerization, while the isomerization decreases with additional excitation of this mode. Excitation of the free OH stretch in the water molecule also increases the amount of isomerization, while excitation of the out-of-plane bending vibration suppresses the isomerization. The effects of partial deuteration on the frequencies for these vibrations are also explored.