THE ORIGIN OF THE PROBLEMS WITH THE PM3 CORE REPULSION FUNCTION

We investigate the cause of failures of PM3 for H ... H, O ... H and N ... H interactions. We show that the actual choice of parameters for the Gaussian correction functions leads to spurious oscillations in th e interatomic core repulsion functions and in the potential energy sur faces. The positions of the minima of the Gaussian correction function s considerably influence the positions of the minima on the potential energy hypersurface of weak interactions. The PM3 potential energy cur ve of water dimer as a function of the O ... H distance clearly illust rates the problem. The minimum at 1.8 (A) over circle and the wide sho ulder at 2.1 (A) over circle correspond to a minimum and a maximum of the O ... H Gaussian correction function, respectively. (C) 1997 Elsev ier Science B.V.