Path-integral molecular dynamics simulations of small hydrated sulfuric acid clusters H2SO4·(H2O)n (n = 1-6) on semiempirical PM6 potential surfaces

A recently developed semiempirical PM6 method was applied to study small hydrated sulfuric acid clusters. Various low-energy structures of the H2SO4·(H2O)n (n = 1-9) clusters were optimized at this level and then compared to previous ab initio and density-functional theory studies in order to understand the applicability of the PM6 method in describing proton-transfer processes as well as hydrogen-bonded structures in the clusters. Although the PM6 method seems to somewhat overemphasize bifurcated hydrogen-bonded structures, moderately good agreement was obtained. Quantum path-integral molecular dynamics simulations for the H2SO4·(H2O)n (n = 1-6) clusters were subsequently performed directly using PM6 potential energies and their gradients. It was found that the acid dissociation probability increases with an increase in the cluster size, as expected, and that so-called contact-ion-pair structures are dominant in the proton-dissociated clusters. The importance of nuclear quantum effects in the cluster structures and proton-transfer processes is demonstrated.