Ground-state correlation effects in molecular photoionization at the extended frozen-core Hartree-Fock level

We present an extension of the frozen-core Hartree-Fock (FCHF) approach to molecular photoionization, allowing us to study the effect of ground-state correlation on the cross sections and photoelectron angular distributions in both a practical and a systematic way. The extended FCHF model is obtained by augmenting the usual FCHF scattering equation by a simple first-order perturbation theoretical expression for the transition moment involving merely bound-state (L2) terms. In addition we describe a numerical method for the treatment of the FCHF scattering problem. Our method makes use of the usual (single-centre) partial wave expansion techniques. The local scattering equations are solved by means of numerical integration and asymptotic matching to the Coulomb (or free) scattering solutions. Subsequently, the non-local exchange potential and the orthogonality constraints are taken into account by using a basis set approach based on the Schwinger variational method. The method has been tested in applications to the photoionization of He, H2, N2(3σg), and CO(5σ). (Author)