AN EFFICIENT POLARIZATION PROPAGATOR APPROACH TO VALENCE ELECTRON-EXCITATION SPECTRA

A practical polarization propagator method devised for the treatment o f valence electron excitations in atoms and molecules is presented. Th is method, referred to as (second-order) algebraic-diagrammatic constr uction (ADC(2)), allows for a theoretical description of single and do uble excitations consistently through second and first order, respecti vely, of perturbation theory. The computational scheme is esentially a n eigenvalue problem of a Hermitian secular matrix defined with respec t to the space of singly and doubly excited configurations. The config uration space is smaller (more compact) than that of comparable config uration interaction (cr) expansions and the method leads to size-consi stent results. The performance of the ADC(2) method is tested in exemp lary applications to Ne, Ar and CO, where detailed comparison can be m ade with experiment and previous theoretical results. While the accura cy of the absolute excitation energies is only moderate, a very satisf actory description is obtained for the relative energies and, in parti cular, for the spectral intensities. Aspects related to the Thomas-Rei che-Kuhn sum rule and the equivalence of the dipole-length and dipole- velocity forms of the transition moments are discussed. Due to the rel atively small computational expense and the possibility of a direct AD C(2) formulation this method should prove particularly useful in appli cations to large molecules.