Configuration interaction and Hylleraas configuration interaction methods in valence bond theory. Diatomic two-electron systems

Abstract

A method to construct configuration interaction (CI) and Hylleraas configuration interaction (Hylleraas–CI) wave functions of the valence bond type from Cartesian Gaussians for diatomic two-electron systems is presented. It is shown that this approach has many advantages in comparison with the usual molecular-orbital (MO) procedure and enables efficient optimization of nonlinear parameters. Sample results for the ground state of H2 are reported. In the case of CI they are better than all CI results obtained previously with Gaussian functions. The presented Hylleraas–CI function gives an error of about 2.8 cm−1 compared with the exact value.