Atomic and Molecular Calculations with the Pseudopotential Method. III. The Theory of Li2, Na2, K2, LiH, NaH, and KH

The pseudopotential theory of diatomic molecules developed in the first part of this series is applied to the calculation of molecular constants for Li2, Na2, K2, LiH, NaH, and KH. Treating these molecules as two-electron problems the core–valence interaction potential is the Hartree–Fock potential plus the exact Phillips–Kleinmann pseudopotential computed in the second part of the series. The wavefunction of the valence electrons is a Heitler–London ansatz for the A2 types and an H–L ansatz augmented by an ionic term containing a correlation factor (1 + cr12) for the hydrides. The computed constants are dissociation energies, equilibrium distances, vibrational frequencies, and dipole moments (for the hydrides). The computated dissociation energies are about 13 of the empirical for the A2 types and 23 of the empirical for the hydrides. The computed equilibrium distances, obtained as the position of the energy minimum, are reasonably close to the empirical in all cases. The results are analyzed, and it is concluded that in the hydrides the charge distribution around the H+ nucleus resembles the electron distribution of an H− atom. A generalization of the pseudopotential method for polyatomic molecules is suggested and its application to large organic molecules is outlined.