STRUCTURE AND DYNAMICS OF WATER AND AQUEOUS-SOLUTIONS - THE ROLE OF FLEXIBILITY

The role molecular flexibility plays in the chemistry of bulk water and ionic solutions is evaluated using molecular dynamics simulations. For simplicity the flexible water model used here is the revised central force (CF) model of Stillinger and Rahman [J. Chem. Phys. 68, 666 (1977)]. A companion, rigid central force (RCF) model is invented to provide the most accurate average description of CF water. Discussion of the role of flexibility is divided into effects due to Coulombic and non-Coulombic parts of the potential energy. The Coulombic part provides the connection between flexibility and nuclear polarizability. A variety of thermodynamic and dynamical properties of the two models are compared. These include the static dielectric constant, orientational correlation functions, self-diffusion coefficient, pressure, intermolecular energy, and pair correlation functions. The effect of flexibility is greatest for dielectric properties, but in general it is found to be small. The dielectric constant of CF water is measured to be approximately 77, in good-perhaps fortuitous-agreement with the experimental value. A variety of ionic solution properties, including the free energy and dynamics of ionic association, are reported for model sodium chloride solutions at two concentrations. For these quantities also the effect of water flexibility is found to be small.