Benchmark Structures and Harmonic Vibrational Frequencies of Hydrated Halide Ions: X– (H2O)n, X = F, Cl, Br, and I (where n = 1 – 4)

This study extends our efforts to generate benchmark structures and harmonic vi- brational frequencies from neutral hydrogen bonded clusters to solvated ions. The analytical gradients and Hessians developed for the N-body:Many-body integrated QM:QM method facilitate the computation of benchmark-quality properties near the CCSD(T) complete basis set limit. In this work, a series of solvated halide ion sys- tems (X−(H2O)n clusters, where X = F, Cl, Br and I and n = 1 to 4) is being characterized with the MP2 and 2-body:Many-body CCSD(T):MP2 (2b:Mb) meth- ods. For n ≥ 2, the latter technique uses the high-level CCSD(T) method to evaluate all 1- and 2-body interactions whereas the low-level MP2 method is used for the 3- body through N-body terms of the many-body expansion. Triple- and quadruple-ζ quality correlation-consistent basis sets were used for these geometry optimizations and harmonic frequency computations. The relative energies, intermolecular OH· · · X bond distances and harmonic vibrational frequency shifts (from the H2O symmetric stretch) are reported for 10 different systems, some of which are found in previous literature and some of which are new structures. The intermolecular OH· · · X bond distances are larger for systems with a larger anion atom and for systems with more waters, and the harmonic vibrational frequency stretches are larger for systems with a smaller anion atom and for systems with fewer waters. Bond distance and vibrational frequency deviations (from benchmark values) demonstrate that method effects play a larger role than basis sets effects when describing these systems