Distorted atoms in molecules (DAM) model for ordinary covalent molecules. II. Force constants for H2, LiH, and BeH+ as well as medium range repulsion of H+H(3Σ/sub u/) and He+He

The distorted atoms in molecules valence-bond model is used to determine potential energy curves near equilibrium for H2, LiH, and BeH+ as well as in the medium range repulsive region for H+H(3Σu) and He+He. It is shown that the force constants that we obtain by numerical differentiation are, in principle, equal to those that would be found by double perturbation theory. The usual (single) perturbation method, however, contains a first-order error. Our values for all force constants including the anharmonic ones lie within 10% of experiment except for k2 of BeH+. The somewhat larger (20%) error in the latter case is probably due to omission of ionic-covalent mixing.