Comparison of classical and quantum statistical mechanical simulations of aqueous ionic clusters

Comparison is made of classical and quantum mechanical treatments of nuclear motion on thermally averaged structural and energetic properties of aqueous clusters. At finite temperatures, radial distribution functions and enthalpies of formation are computed using classical and path integral Monte Carlo simulations. Diffusion Monte Carlo simulations are used to calculate quantum mechanical enthalpies of formation at 0 K. Calculations are reported for Cl-(H2O)(n) and I-(H2O)(n) with n = 1-6, for temperatures from 0 to 300 K. Comparisons are also made with properties computed within the harmonic approximation, which is often used to extend electronic structure calculations to finite temperatures, using the geometries, energies, and frequencies at minimum-energy geometries. It is found that the quantum mechanical results can differ markedly from the classical ones, particularly at the lower temperatures. We discuss the implications of these findings for obtaining analytical fits to potential energy surfaces and for interpreting experimental investigations into cluster structure.