AN EFFICIENT STRING-BASED TRUNCATION SCHEME FOR MCSCF CALCULATIONS

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 trunca ted determinantal expansions do not necessarily yield spin-eigenfuncti ons, we evaluate [(S) over cap(2)] for all examples considered and sho w that the level of spin-contamination is negligibly small. This type of approach makes it feasible to contemplate MCSCF calculations with a bout 20 active electrons and orbitals. (C) 1998 Elsevier Science B.V. All rights reserved.