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 the interatomic core repulsion functions and in the potential energy surfaces. The positions of the minima of the Gaussian correction functions considerably influence the positions of the minima on the potential energy hypersurface of weak interactions. The PM3 potential energy curve of water dimer as a function of the O ... H distance clearly illustrates the problem. The minimum at 1.8 (A) over circle and the wide shoulder at 2.1 (A) over circle correspond to a minimum and a maximum of the O ... H Gaussian correction function, respectively. (C) 1997 Elsevier Science B.V.