Electron correlation effects in ionic hydrogen clusters

We employ density functional, post-Hartree-Fock, and quantum Monte Carlo methods to study the electronic structure, geometries, and behavior of positively charged H-m(+) clusters with in = 3,5,...,17. Their structure consists of a tightly bound H-3(+) core ion surrounded by successive solvation shells of H-2 molecules. For the largest clusters, we propose new geometries. We find that correlated methods yield the stepwise decrease of enthalpies for dissociation of H-2 from the clusters observed in experiments. Our best results are obtained by the diffusion Monte Carlo method, and by including finite temperature entropic effects, we are able to reproduce the experimental dissociation enthalpies with an unprecedented accuracy of less than 0.5 kcal/mol. These benchmark results contrast with erroneous predictions discovered in the density functional approaches. Finally, our analysis of the cluster energy surfaces indicates that under quantum and thermal fluctuations, the outer solvation shells will exhibit pronounced fluctional behavior. (C) 2001 John Wiley & Sons, Inc.