Computational quantum chemistry is used in catalysis research in ways that are very different from those in theoretical chemistry research, and as a consequence, standard theoretical chemistry approaches are sometimes not applicable. This paper considers issues associated with the study of heterogeneous catalytic sites and intermediates where single reference theories are not sufficiently accurate, and therefore a technique like multi-configurational self-consistent field (MCSCF) theory must be utilized. The geometric and electronic structure of heterogeneous catalyst sites and intermediates are often not known or are ambiguous, and in such situations, standard theoretical chemistry approaches may be unsuitable. In this paper, the issues involved in using MCSCF for molecular catalysis work are examined. Iron pentacarbonyl and ferric chloride dimer are used as test molecules to illustrate cases where Hartree-Fock (HF) and density functional theory (DFT) both show deficiencies. These test molecules are then used to illustrate how MCSCF can be applied without prior knowledge of geometric or electronic structure and without the benefit of supporting experimental information, to obtain more accurate results than those from the single reference methods. (C) 2002 Elsevier Science B.V. All rights reserved.