The Use of the Hellmann–feynman Theorem to Calculate Molecular Energies

The Hellmann–Feynman theorem has been employed to calculate the repulsion between two helium atoms and the molecular energies of H2 and H3. The method of molecular orbitals was used to determine the necessary expressions for the electron densities. The screening constants of the atomic orbitals comprising the molecular orbitals were treated as functions of the internuclear separation according to an empirical expression which duplicates very closely the "best" values for these parameters as determined by the variational method. The results of the calculations indicate that the present method is capable of yielding estimates of molecular energies which are comparable to those obtained by the more elaborate and time-consuming variational methods.