DYNAMICAL FLUCTUATING CHARGE FORCE-FIELDS - APPLICATION TO LIQUID WATER

A new molecular dynamics model in which the point charges on atomic si tes are allowed to fluctuate in response to the environments developed and applied to water. The idea for treating charges as variables Is b ased on the concept of electronegativity equalization according to whi ch: (a) the electronegativity of an atomic site is dependent on the at om's type and charge and is perturbed by the electrostatic potential i t experiences from its neighbors and (b) charge is transferred between atomic sites in such a way that electronegativities are equalized. Th e charges are treated as dynamical variables using an extended Lagrang ian method in which the charges are given a fictitious mass, velocitie s, and kinetic energy and then propagated according to Newtonian mecha nics along with the atomic degrees of freedom. Models for water with f luctuating charges are developed using the geometries of two common fi xed-charge water potentials: die Simple point charge (SPC) and the fou r-point transferable intermolecular potential (TIP4P). Both fluctuatin g charge models give accurate predictions for gas-phase and liquid sta te properties, including radial distribution functions, the dielectric constant, and the diffusion constant. The;method does not introduce a ny new intermolecular interactions beyond those already present in the fixed charge models and increases the computer time by only a factor of 1.1, making this method tractable for large systems.