The multiconfigurational spin-tensor electron propagator method for determining vertical principal and shake-up ionization potentials for open shell and highly correlated atoms and molecules

We propose and develop the multiconfigurational spin-tensor electron propagator (MCSTEP) technique for the theoretical determination of vertical ionization potentials (IPs) and electron affinities (EAs) for general open-shell and highly correlated atoms and molecules. We obtain these equations from a Green’s function or electron propagator approach where we properly couple electron removal and addition tensor operators to a multiconfigurational tensor state. To account for important shake-up effects and to achieve a ‘‘balance’’ in initial and final state correlation corrections, we include in MCSTEP ionization and electron affinity operators analogous to the ‖c〉〈0‖ state transfer operators necessary in multiconfigurational linear response. In repartitioned MCSTEP (RMCSTEP) we augment the MCSTEP operator manifold with operators of the form a+iajak by first employing partitioning theory to estimate their contributions and then repartitioning only the important operators into the primary space. In this way, important shake-up processes to diffuse orbitals are accurately and reliably handled with RMCSTEP at the same level of approximation, i.e., as part of the primary space operator manifold . Initial application of these methods is extremely encouraging for both principal and shake-up IPs. Using a 〈5s5pld〉 contracted Gaussian valence basis set augmented with two diffuse s, two diffuse p, and two diffuse d functions, the RMCSTEP ionization potentials to the low-lying (<∼24 eV) 2S and 2P bound ionic states (including diffuse states) for Be are calculated within ±0.07 eV of experiment. The IP to the lowest 2D state is calculated 0.14 eV from experiment.