ON THE CHOICE OF THE ACTIVE SPACE AND THE BASIS-SET FOR MCSCF CALCULATIONS ON SMALL MOLECULES AND REACTIVE SURFACES

A systematic study of the active space in MCSCF calculations, using N-2 and the stationary points on the HCN --> CNH and HCCH --> CCH2 potential hypersurfaces as-model systems, is reported. The different sets of active orbitals are tested with a variety of standard basis sets. The optimized geometries of the ground states and transition states are compared with experiment and other theoretical studies. It is concluded that MCSCF calculations with two occupied and two virtual orbitals (MCSCF(2+2)), in which the active space is formed from the 1 pi(u)(4) and 1 pi(g)* orbitals, provide highly accurate descriptions of the equilibrium ground-state geometries of N-2, HCN, CNH, and HCCH. In particular, the results in which a split-valence + polarization basis is employed are very close to the experimental values. For the hydrogen rearrangement reactions, however, an active space comprising four occupied and four virtual orbitals (MCSCF(4+4)), together with a large basis, is required for a well-balanced potential surface. The barriers toward hydrogen migration are in these cases in very good agreement with experiment, and with previous CISD and CCSD studies.