The density of a molecule expanded in plane waves is decoupled from its periodic images using a fit to atom-centered Gaussians, which reproduces the long-range electrostatic potential of the original density. The interaction energy between the cluster and its periodic images is calculated by an Ewald summation. The method has been applied to self-consistent cab initio molecular dynamics calculations of charged and polar molecules. An atomic point charge model of the charge density is obtained, which can be used for classical molecular dynamics models and to couple classical and quantum mechanical simulations.