Application of unitary group methods to configuration interaction calculations

A computational approach to the direct configuration interaction method is described. The method is formulated using the calculus of the generators of the unitary group. The simple structure of the generator matrices within the harmonic excitation level scheme is exploited to give a computational method that is competitive with traditional approaches. A new scheme for basis set truncation in the case of partial configuration interaction is devised employing orbital populations. It is also shown that the block structure of the generator matrices leads to the definition of a new order parameter for perturbation methods which is both effective and convenient.