Hybrid Treatments Based on Determinant Seniority Numbers and Spatial Excitation Levels in the Configuration Interaction Framework

DIEGO R. ALCOBA; OFELIA B. OÑA; LUIS LAIN; PABLO CAPUZZI; ALICIA TORRE; GUSTAVO E. MASSACESSI

Novel Electronic Structure Theory: General Innovations and Strongly Correlated Systems, A Special Volume of Advances in Quantum Chemistry (Series)

Elsevier Academic Press Inc.

Año: 2018; p. 315 - 331

In this work we project the Hamiltonian of an N-electron system onto a set of N-electron determinants cataloged by their seniority numbers and their excitation levels with respect to a reference determinant. We show that, in open-shell systems, the diagonalization of the N-electron Hamiltonian matrix leads to eigenstates of the operator S^2 when the excitation levels are counted in terms of spatial orbitals instead of spin-orbitals. Our proposal is based on the commutation relations between the N-electron operators seniority number and spatial excitation level, as well as between these operators and the spin operators S^2 and Sz. Energy and expectation values of molecular systems obtained from our procedure are compared with those arising from the standard hybrid configuration interaction methods based on seniority numbers and spin-orbital-excitation levels. We analyze the behavior of these methods, evaluating their computational costs and establishing their usefulness.