A representation of the polarization term in the interaction energy between a molecule and a point-like charge

The interaction energy of a molecule M with a point-like charge q can be partitioned into simpler contributions, two of which can be expressed in terms of the charge distribution _M of the sole M. The first term, qV(r), represents the interaction of q with the undistorted charge _M⁰ of M while the second q²P(r) gives the additional contributions due to the polarization of _M⁰ under the influence of the charge q placed at the point r. In this paper we investigate the possibility of getting an inexpensive and sufficiently accurate analytical representation of P(r) over the whole space outside the van der Waals volume of M.