The fluctuating charge (FQ) model proposed by Rick et al. [(J. Chem. Phys. 101, 6141 (1994)] for molecular dynamics (MD) simulation of water is applied to a test case for polyatomic ionic systems. A system resembling alkali cyanide crystals, with two partial charges on the atomic sites of the polarizable anions, is considered. The need for charge fluctuation considerations in such a simple system is demonstrated by ab initio calculations of the partial charges in the cyanide ion with different orientations within a fixed octahedral environment of sodium ions. It is shown that the charge distributions 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 shrink with increasing repulsive short-range interactions. Furthermore, a well-known polarization effect, that is, the reduction in the frequencies of longitudinal optic modes of the crystal, is also obtained with the FQ model.

Ab initio calculations have been performed in order to investigate a recently proposed polarizable model [M. C. C. Ribeiro, Phys. Rev. B 61, 3297 (2000)] for molecular dynamics (MD) simulation of the molten salt Ca0.4K0.6(NO3)1.4. On the basis of the electronegativity equalization method, polarization effects in the MD simulations have been introduced by a fluctuating charge (FC) model for the nitrate ion. Partial charges in the nitrate ion are obtained by ab initio calculations at several levels of theory, and compared with previously proposed models for MD simulations of nitrate melts. Charge fluctuation is achieved in the ab initio calculations by using positive probe charges placed around a nitrate ion. The parameters of the FC model are corroborated by comparison of the ab initio partial charges with the ones obtained directly by the electronegativity equalization method. Simulated annealing of a cluster including two double-charged cations and two nitrate ions shows that very different structures are obtained depending on whether the FC model or its nonpolarizable counterpart is considered. Ab initio calculations show that the structure of this cluster is strongly dependent on polarization effects in the nitrate ions.

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