Diffusion Monte Carlo simulations on uracil-water using an anisotropic atom-atom potential model

We have developed an anisotropic atom-atom intermolecular potential model for the interaction of uracil with water. The potential consists of a distributed multipole analysis (DMA) model for the electrostatic energy, and a 6-exp potential to represent the repulsion-dispersion term. The repulsion-dispersion potential parameters are adjusted to yield good agreement with accurate ab initio data on the minima and transition states of the uracil-water complex. We have used this potential in diffusion Monte Carlo simulations of uracil-water, uracil-(water)2 and uracil-(water)3. The uracil-water simulations show that the theoretically based potential gives a qualitatively different picture of uracil hydration than that provided by a standard isotropic atom-atom point charge model, which is shown to underestimate the delocalized motion of the water hydrogen atoms. Plots of the vibrational probability density of the hydrogen atoms show the delocalized motion of the water hydrogen atoms that are not involved in hydrogen bonding.