Charge distribution from a simple molecular orbital type calculation and non-bonding interaction terms in the force field MAB.

A simple and fast method to calculate charge distributions in organic molecules is presented. The method is based on charge shifts within the saturated sigma-system, driven by orbital electronegativities, coupled to a modified Hückel treatment of the unsaturated pi-systems. Experimental molecular dipole moments of a set of 119 molecules are reproduced with a root mean square deviation of 0.36 Debye units. Furthermore, the obtained charge distribution is used to describe hydration free energies in terms of hydrogen-bonding donor and acceptor strengths of polar groups. Least square fitting to experimental data of 281 compounds leads to values for these strengths with accuracy limits of +/- 4.3% and +/- 2.5%, respectively. Properly normalized values are taken to parametrize the hydrogen bonding terms in our MAB force field. The method is sufficiently fast to be used in the preparatory phase of interactive force-field calculations.