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.