Variation–Perturbation Treatment of Electron Correlation in Molecules. I. Bare Nucleus Model for H2

A variation–perturbation (VP) calculation of the electronic energy of H₂ is carried out using $r_{12}^{\text{−\hspace{0.17em}1}}$ as the perturbation. The first‐order wavefunction gives a total energy through third order which is only 0.4 kcal/mole greater than the exact value. It is seen that the VP energies are relatively insensitive to errors in the unperturbed wavefunction and that accurate results can be obtained with limited variational basis sets of only two or three functions. Application of either the bare nucleus or a similar model to more complicated molecules is discussed.