An efficient string-based truncation scheme for MCSCF calculations

Abstract

We describe an effective truncation scheme for reducing the dimension of the determinantal space in multiconfiguration self-consistent field (MCSCF) calculations. The range of validity of the scheme is explored through calculations of N-electron/ N-orbital and N-electron/ M-orbital ( M> N) active spaces and comparison with exact results. Since truncated determinantal expansions do not necessarily yield spin-eigenfunctions, we evaluate 〈 S ̂ 2〉 for all examples considered and show that the level of spin-contamination is negligibly small. This type of approach makes it feasible to contemplate MCSCF calculations with about 20 active electrons and orbitals.