The Configuration Interaction Method

Abstract

The simplest ab initio quantum chemical method is the Hartree-Fock method which has been outlined in detail in a preceding chapter. Other names for this method are the Self Consistent Field (SCF) method and the independent particle method. Due to its simplicity the Hartree-Fock method has become a very useful tool for stable molecules around their equilibrium geometries. The Hartree-Fock total energy differs from the exact non-relativistic energy by an energy, which for historical reasons is called the correlation energy. There are essentially two shortcomings of the Hartree-Fock method which sometimes make the correlation energy large. The most important of these shortcomings is connected with how the Hartree-Fock method is normally used, namely with the restriction that two electrons are placed in each spatial orbital, the Resticted Hartree-Fock (RHF) procedure. This restriction is responsible for, for example, the very large error of several eV obtained at the asymptotic limit when the hydrogen molecule is dissociated. Other examples are given in the chapter on the Multi- Configuration SCF (MCSCF) method. As mentioned in that chapter the problem with improper dissociation is corrected by the use of a few additional determinants (or configurations). For this reason this error is said to be due to near-degeneracy effects.