Quantum simulations of aqueous systems

Abstract

Discretized path-integral simulation methods have been applied to the determination of structure in two quantum mechanical aqueous systems. The first of these applications is the determination of the consequences of quantizing the rigid-body degrees of freedom of the water molecules in the many-particle pure room temperature liquid. The results provide a quantitative estimate of the significance of approximating such a system as classical and also of the size of isotope effects on the liquid structure. These features are found to have a close analogue in the structural response of the fluid to temperature. Second, we consider the structure of a hydrated excess electron. Here we treat the water classically but treat the highly quantum mechanical electron via a path-integral description, introducing a local electron-water pseudopotential for the interaction. The excess electron density and solvent distribution are examined and shown to exhibit strong structural similarities to ionic solvation. However, it is found that the electronic density fluctuates sufficiently in size and shape as to nearly erase distinct features in the electron-solvent radial correlations. For both of the aqueous systems considered, comparison of results following from the simulations with experimentally accessible direct structural measures yields satisfactory agreement.