Fluctuating charge model for polyatomic ionic systems: A test case with diatomic anions

The fluctuating charge (FQ) model proposed by Rick et al. [(J. Chem. Phys. 101, 6141 (1994)] for molecular dynamics (MD) simulation of water is applie d to a test case for polyatomic ionic systems. A system resembling alkali c yanide crystals, with two partial charges on the atomic sites of the polari zable anions, is considered. The need for charge fluctuation considerations in such a simple system is demonstrated by ab initio calculations of the p artial charges in the cyanide ion with different orientations within a fixe d octahedral environment of sodium ions. It is shown that the charge distri butions in the crystal obtained with the FQ model are sensitive to changes in the environment in such a way that the anions become more polarizable as the lattice parameter increases. Conversely, the charge distributions shri nk with increasing repulsive short-range interactions. Furthermore, a well- known polarization effect, that is, the reduction in the frequencies of lon gitudinal optic modes of the crystal, is also obtained with the FQ model. ( C) 1999 American Institute of Physics. [S0021-9606(99)50423-6].