On the performance of molecular polarization methods close to a point charge

The induced dipole moment of a water molecule close to a point charge (with the radius of Na+ or Be2+) is computed with the three main methods to implement molecular polarization (point dipoles, fluctuating charges and shell model). The results are compared with high level ab initio calculations and studied as a function of the charge-molecule distance for selected molecular orientations. For the single charge case rather good estimations of the ab initio induced dipole are obtained, with no significant divergences appearing at the shortest distances. Nevertheless, these (linear polarization) methods fail as the charge increases, suggesting that nonlinear effects may not be neglected. Regarding the capabilities of each method, the point dipole method is the one that performs better overall, with the shell model achieving acceptable results in most instances. The fluctuating charge method has some noticeable limitations for implementations with a similar number of interaction sites.