Multireference perturbation theory applied to multiconfigurational problems: is the CASSCF step necessary?

A no. of groups have suggested that CASSCF orbitals are not essential in applying multireference methods, and that orbitals generated from simpler methods (Hartree-Fock or Kohn-Sham) might serve as well. In this study, we investigate the MRMP2 method with a variety of different orbital sets and assess the reliability of such procedures. The MRMP2 method, and many similar variants of multireference perturbation theory have a wide range of applicability and it is worthwhile to investigate whether 'simpler' orbitals than those obtained from the full CASSCF procedure can provide useful accuracy. We are particularly interested in systems which require a multiconfigurational description over most or all regions of the potential energy surface. In this study we investigate the potential energy curves of the X1.sum.+g, B1Delta g and B'1.sum.+g states of the C2 mol. These are demanding multiconfigurational test cases for which full CI results have been recently published. We compare our calcns. with these benchmark results. For the mapping of complete potential energy curves we find that the CASSCF orbitals are essential, but that orbitals obtained from simpler techniques can yield reliable data in restricted regions (out to approx. 1.5Re) of the potential energy curves. [on SciFinder (R)]