ANALYSIS OF THE CORE-REPULSION FUNCTIONS USED IN AM1 AND PM3 SEMIEMPIRICAL CALCULATIONS - CONFORMATIONAL-ANALYSIS OF RING-SYSTEMS

The AMI and PM3 potential energy surfaces of cyclopentane have been compared and the energy difference between two (boat and chair) cyclohexane conformers analyzed. The effects of reparameterization and different core-repulsion functions (CRFs) have been studied. The H-H, C-H, and C-C CRF energy contributions have been calculated separately. The results show that the CRF energy corrections stabilize the correct (MM2 and ab initio) geometries. The PM3 equilibrium geometry of cyclopentane is closer to the MM2 and ab initio results than the AM1 geometry. It is known that above 190 pm the AM1 H-H CRF curve is smooth and negative but the PM3 H-H CRF curve has a trough and a shoulder. It has been shown that this latter oscillation is not damped by other parameters. As a result, the H-H PM3 energy gradient curve is different from the quasilinear AM1 and ab initio gradient curves. (C) 1993 by John Wiley & Sons, Inc.