Many-body polarization point charge

Numerous polarizable water models have recently been developed [75-82]. At least three types of polarizable models have been used for supercritical water PPC [83], a polarizable TIP-type model [84], and a few variations of SPC-type models with either point or smeared electrostatic charges [78,85,86]. With a few exceptions [87-89] the polarizable models are typically built upon successful non-polarizable counterparts, by scaling the Coulombic charges to match the gas-phase dipole moment, and by including either a polarizable point charge or point dipole to account for the many-body polarization contributions. Moreover, sometimes the permanent dipole moment is set larger than the gas-phase value of 1.85D in order to obtain better agreement with experimental data at ambient conditions [78,82]. 

It is worth mentioning that preliminary consideration of MM scheme has resulted already in some doubts and objections. Generally speaking, the classical description of the essentially quantum molecular systems cannot be exact and fuU. Most of the terms in O Eqs. 9.2-9.S refer to the first approximation or to the first term of expansion of the corresponding interaction energy. The atoms are not points, they have dipole and quadrupole moments (not only charges), charge distribution in a molecule is continuous, the polarization or electron delocalization interactions are not considered in the classical minimalist MM approach, the contributions of three-body and four-body interactions can be essential ones. Many attempts have been undertaken to overcome these inherent difficulties of the MM method as well as to justify the assumptions and simplifications we will consider some of these attempts below. Few remarks for justification of the main principles of MM method are described here. 

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