Benchmark Electronic Structure Calculations for H3O+(H2O)n, n=0-5, Clusters and Tests of an Existing 1,2,3-Body Potential Energy Surface with a New 4-Body Correction

We report extensive benchmark CCSD(T) Complete Basis Set (CBS) estimates of the binding energies, structures, and harmonic frequencies of H3O+(H2O)(n) clusters, n = 0-5, including all currently known low-lying energy isomers. These are used to test a previously reported many body (up to 3-body interactions) CCSD(T)-based potential energy surface (PES) for the hydrated proton. A new 4-body term for the hydronium-water-water-water interactions is introduced. This term is aimed at refining the relative energies of isomers of the H3O+(H2O)(n), n = 4, 5 clusters. The test results of the revised PES against the benchmark demonstrate the high accuracy of the revised PES.