Comparing polarizable force fields to ab initio calculations reveals nonclassical effects in condensed phases

In a recent work [Giese and York J. Chem. Phys. 120, 9903 (2004)] showed that many-body force field models based solely on pairwise Coulomb screening cannot simultaneously reproduce both gas-phase and condensed-phase polarizability limits. In particular, polarizable force fields applied to bifurcated water chains have been demonstrated to be overpolarized with respect to ab initio methods. This behavior was ascribed to the neglect of coupling between many-body exchange and polarization. In the present article we reproduce those results using different ab initio levels of theory and a polarizable model based on the chemical-potential equalization principle. Moreover we show that, when hydrogen-bond (H-bond) forming systems are considered, an additional nonclassical effect, i.e., intermolecular charge transfer, must be taken into account. Such effect, contrarily to that of coupling between many-body exchange and polarization, makes classical polarizable force fields underpolarized. In the case of water at standard conditions, being H-bonded geometries much more probable than the bifurcated ones, intermolecular charge transfer is the dominant effect.