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

D.R. ALCOBA; A. TORRE; L. LAIN; O.B. OÑA; G.E. MASSACCESI; P. CAPUZZI

Buenos Aires

Congreso; Molecular Electronic Structure in Buenos Aires (MESBA 2016); 2016

Consejo Nacional de Investigaciones Científicas y Tecnológicas

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.