Magic numbers, quantum delocalization, and orientational disordering in anionic hydrogen and deuterium clusters

The Diffusion Monte Carlo (DMC) method was applied to anionic hydrogen clusters H-(H-2)(n) (n = 1-16, 32) and their deuterated analogs using a polarizable all-atom potential energy surface (PES) developed by Calvo and Yurtsever. For the hydrogen clusters, the binding energy Delta E-n appears to be a smooth function of the cluster size n, thus contradicting the previous claim that n = 12 is a magic number cluster. The structures of the low energy minima of the PES for these clusters belong to the icosahedral motif with the H-2 molecules aligned toward the central H- ion. However, their ground state wavefunctions are highly delocalized and resemble neither the structures of the global nor local minima. Moreover, the strong nuclear quantum effects result in a nearly complete orientational disordering of the H-2 molecules. For the deuterium dusters, the ground state wavefunctions are localized and the D-2 molecules are aligned toward the central D- ion. However, their structures are still characterized as disordered and, as such, do not display size sensitivity. In addition, DMC simulations were performed on the mixed H- (H-2)(n) (D-2)(p) clusters with (n, p) = (6, 6) and (16, 16). Again, in contradiction to the previous claim, we found that the more quantum H-2 molecules prefer to reside farther from the central H- ion than the D-2 molecules. Published under license by AIP Publishing.