SOME DIFFICULTIES ENCOUNTERED WITH AM1 AND PM3 CALCULATIONS

AM1 and PM3 underestimate frontier intel actions with respect to steri c repulsions. Therefore, if two structures differ by (similar to)1 kca l/mol, their calculated ordering is unreliable. Activation energies te nd to increase with substitution, regardless of electronic effects. At omic charges are sometimes unrealistic tin enolates, the negative char ge is larger on C than on O). At van der Waals distances, acid-base an d coulombic interactions can prevail over steric repulsions. At all di stances, basicities are overestimated and nucleophilicities underestim ated. This may lead to anomalous ion-molecule and transition structure s in gas phase reactions. Transition structures are tighter than in ab initio calculations. Optimisations may give chemically unreasonable s tructures. Minimum energy paths are then difficult to obtain. Usually, but nor systematically, PM3 gives more reliable structures and AM1 mo re realistic energies. (C) 1998 Elsevier Science Ltd. Ail rights reser ved.