Molecular spinors suitable for four-component relativistic correlation calculations: Studies of LaF⁺ and LaF using multiconfigurational quasi-degenerate perturbation theory

Multiconfigurational second-order quasidegenerate perturbation theory (MCQDPT) calculations were performed for the LaF⁺ molecule, with one LaF²⁺ and four LaF⁺ Dirac-Fock-Roothaan (DFR) spinor sets. The best spinor set was that of LaF²⁺, which gave the lowest total energies and also the best excitation energies for any state considered. The MCQDPT calculations with the cation and neutral molecular spinors were also performed for LaF. The MCQDPT with the cation spinors gave the lowest total energies for all states under consideration, and the calculated excitation energies compared best with experiment. We prefer the LaF⁺ spinor set to those of LaF. These calculations indicate that the DFR spinor set for the (n-1) electron system is adequate for treating the molecular electronic system having n electrons.