Configuration Interaction Wave Functions: Excitation And Seniority Number Based Approaches

D.R. ALCOBA; A. TORRE; L. LAIN; G.E. MASSACCESI; O.B. OÑA; P. CAPUZZI; M. VAN RAEMDONCK; W. POELMANS; S. DE BAERDEMACKER; D. VAN NECK; P. BULTINCK

Buenos Aires

Conferencia; Molecular Electronic Structure Buenos Aires Conference (MESBA 2016); 2016

Consejo Nacional de Investigaciones Cientificas y Tecnicas

Although the N-electron wave functions arising from the full configuration interaction (FCI) treatment are the exact solutions of the Schrödinger equation for a given basis set, their determination usually requires a great computational effort. To reduce this cost one formulates expansions of configuration interaction (CI) in which only a limited number of N-electron Slater determinants is taken into account. The most popular criteria to select these determinants are based on i) the particle-hole excitation level from a reference determinant and ii) the seniority number of the determinants1,2. In this work we study the spin contamination of the CI N-electron wave functions provided by both approaches by means of the excitation operator and the seniority number operator, respectively. We show that the excitation based procedure leads to eigenfunctions of the spin squared operator if the excitations are counted in terms of spatial orbitals while the seniority number based method always provides pure s in states. Hybrid CI treatments3 based on both seniority number and excitation schemes have  also been  investigated, showing  its adequacy to describe systems which exhibit dynamic correlation at determined geometrical arrangements but show static correlation at other arrangements. References [1] L. Bytautas, T.M. Henderson, C.A. Jiménez-Hoyos, J.K. Ellis, and G.S. Scuseria, J. Chem. Phys. 135, 044119 (2011). [2] D.R. Alcoba, A. Torre, L. Lain, G.E. Massaccesi, and O.B. Oña, J. Chem. Phys. 139, 084103 (2013). [3] D.R. Alcoba, A. Torre, L. Lain, O.B. Oña, P. Capuzzi, M. Van Raemdonck, P. Bultinck, and D. Van Neck, J. Chem. Phys. 141, 244118 (2014).