Atomic charges in molecular mechanical force fields: A theoretical insight

Based on quantum theory of the Coulombic interactions between a molecule and its surrounding molecules, a theoretical derivation is presented to obtain an atomic charge model. The charge model shows that the appropriate atomic charge, used for example in molecular mechanical force fields, is simply the average value of the electrostatic potential (ESP) derived charge for an isolated molecule and that obtained for the molecule in the interaction system. Computational scheme to calculate the ESP derived atomic charges of a molecule in the interaction system is presented. The method is applied to two common liquids with associated hydrogen bonds: water and methanol. The obtained theoretical atomic charges are similar to those found in the common interaction potential models, such as: SPC, TIP3P, OPLS, etc. However, for methanol they differ considerably from those obtained using the restrained ESP method.