Compactness Of Full Configuration Interaction Wave Functions: A Seniority Number Approach

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

Isla de San Cristobal

Congreso; 40th Congress Of Theoretical Chemists Of Latin Expression - Chitel Xl; 2014

Universidad San Francisco de Quito

The concept of seniority number, widely used to classify Slater determinants, is extended to N-electron wave functions of spin S, as well as to N-electron spin-adapted Hilbert spaces. A spin-free formulation of the seniority number operator allows us to determine the invariant character of the expectation value of this operator for spaces of N electrons and spin S and its non-invariant character under unitary transformations for a determined wave function. We study the convergence of the full configuration interaction wave function expansions in terms of Slater determinants, using three types of basis sets, i.e. i) the canonical molecular orbitals, ii) the natural orbitals for the corresponding wave function and iii) the orbitals which minimize the expectation value of the seniority number operator for that wave function. The results found show that the last expansions exhibit a superior convergence and that the expectation value of the seniority number operator provides a quantitative procedure to evaluate the compactness of the wave functions [1]. References: [1] Alcoba, D.R., Torre, A., Lain, L., Massaccesi, G.E., Oña, O.B. J. Chem. Phys. 2013. 139, 084103