Molecular dynamics simulations of water with novel shell-model potentials

The need for a computationally efficient yet physically realistic water mod el for molecular simulation is longstanding. To account for intermolecular interactions with other molecules, for example the hydration of biomolecule s, the model has to be able to adapt itself to different environments. To t his end, a number of water models have been proposed that incorporate polar izability with varying degrees of success. We have developed new water mode ls, flexible as well as rigid, on the basis of the shell concept, with thre e atoms and an extra particle representing the electronic degrees of freedo m. This particle is coupled to a dummy position on the bisector of the mole cule by a harmonic spring. We named the models SW, or shell water models. T o account for anisotropic polarization, the spring force on the shell parti cle has been modified to depend on the displacement of the shell particle i n the molecular coordinate frame. The model is constructed so as to reprodu ce the vacuum dipole and quadrupole, and the spring constants are chosen su ch that the polarizability, be it isotropic or anisotropic, is consistent w ith experimental data. Then the Lennard-Jones parameters are optimized to r eproduce the Liquid energy and density. The model is evaluated by checking liquid- and gas-phase properties, and the influences of anisotropy and flex ibility are discussed.