Charge calculations in molecular mechanics. Part 8. Partial atomic charges from classical calculations.

The CHARGE2 programme, which involves the classical calculation of both the inductive and resonance contributions to the partial atomic charges in molecules is described, and the charges and electrostatic potentials obtained presented for some illustrative examples. In substituted methanes (CH3X, CF3X, CCl3X) the effects of varying the electronegativity of the substituents and the alpha- and beta-substituent contributions are clearly illustrated for a variety of substituent groups X. The problems involved in the inclusion of silicon into this scheme are detailed, together with the methods of overcoming them. The partial atomic charges (sigma and pi contributions) and electrostatic potentials for some silicon oxygen compounds are presented and discussed. The partial atomic charges from CHARGE2 for all the natural amino acids as their N-acetyl, N'-methylamides are given and compared with those obtained from the AMBER and ECEPP/2 force fields. Considerable differences in these figures are observed, with the AMBER charges consistently much larger than those from the other two methods. The CHARGE2 partial atomic charges and electrostatic potentials for the four common nucleic acids, adenine, cytosine, guanine and thymine, are given and compared with those derived from other calculations. Again there is general similarity but also there are considerable differences, with those from the AMBER force field somewhat larger than the other methods.