Diffusion Monte Carlo Studies of Small Clusters
We explored the energetics, structural properties, and nuclear quantum effects of molecular and atomic clusters, namely water, halide ion-water, and para-hydrogen along with their corresponding deuterated isotopologues. These systems typically have very complex potential energy landscapes with numerous minima corresponding to various geometric motifs and isomers, thus making numerical simulations challenging. Low energy barriers in combination with nuclear quantum effects may cause the ground state wavefunction to spread and become delocalized over more than one minimum in the configuration space. Moreover, isotope substitution may lead to marked structural changes and isotope effects in these systems involving different types of isomers, or even to quantum melting. Often, the ground state wavefunctions of the most quantum isotopologues have appreciable amplitude in several or in numerous potential energy minima, while those of the more classical isotopologues are primarily localized either to a single minimum (i.e., the global minimum) or to a much smaller subset of competing local minima separated by high potential energy barriers. Essentially, all of the aforementioned studies were carried out using the diffusion Monte Carlo (DMC) method, which, in principle, provides numerically exact results for the ground state energies and wavefunctions of nontrivial, many-body systems.