Quantum simulations of water clusters: Elucidating many-body forces in the liquid.

摘要

This thesis details the first quantum simulations of water clusters and simulations of liquid water using a polarizable potential energy surface fit to water dimer spectroscopic data. Provided is a review of theoretical studies of water, in which we discuss the perturbation theory derivation of the analytical expressions for the long range and short range forces, and then detail the development of ab initio potentials for the study of the water dimer and trimer interactions.; The determinations of the spectroscopic VRT(ASP-W)II and III potential energy surfaces are presented. These represent the second and third fitting of the ASP-W ab initio potential, using Leforestier's Pseudo-spectral Split Hamiltonian approach (PSSH). In PSSH, the six-dimensional dimer Hamiltonian is diagonalized and exchange-repulsion parameters of the potential are fit via a non-linear least squares fitting routine so as to reproduce experimentally observed microwave and terahertz transitions. Both potentials are presented because the accuracy of VRT(ASP-W)II has been tested much more extensively, and VRT(ASP-W)III represents further refinement of the potential, but otherwise is very similar. VRT(ASP-W)II and III are both able to reproduce (H₂ O)₂ and (D₂O)₂ spectroscopic data to a high level of accuracy. VRT(ASP-W)III is then used to calculate the canonical partition function for (H₂O)₂, so as to determine the equilibrium constant for dimerization, K_P( T), and to resolve ambiguity over (H₂O)₂ atmospheric concentrations. The determinations of additional thermodynamic properties, Δ G, ΔH, ΔS, C_P, C_V, for (H₂O)₂ are presented. In addition, the role of quasi-dissociative states in the calculation of K_P(T) are discussed at length.; Next, we determine the vibrational ground-states of larger water clusters using VRT(ASP-W)II and VRT(ASP-W)III in Diffusion Quantum Monte Carlo (DQMC) simulations. The results from VRT(ASP-W)II and III are compared to those from the ab initio ASP-W potential as well as several bulk potentials. VRT(ASP-W)III is shown to be the most accurate model for the vibrational ground-states of larger water clusters to date. The effects of the fits are assessed and the importance of many-body induction and three-body dispersion are examined, and analytical forms for three-body exchange are discussed. VRT(ASP-W)III is then used in simulations of liquid water, and the resulting structural and energetic data are presented. Comparison is made to results with several other potential energy surfaces of similar spectroscopic accuracy. We then discuss the role of many-body induction, three-body exchange, three-body dispersion and intramolecular flexibility in the accurate determination of the liquid water structure. Finally, suggestions are made for future work on the determination of a “universal” potential for water.