ABSTRACT
Molecules containing transition metals are traditionally considered to be difficult to treat by ab initio methods. Apart from the fact that these systems are often so large that their correlation problem involves many electrons (leading, for exam ple, to problems with size extensivity), it was realized as soon as the first applica tions began to appear that the occurrence of open shells, together with occasion ally strong near-degeneracies, made it impossible to obtain the same kind of accuracy for first-row transition metal systems as could be obtained for small organic molecules using rather simple (e.g., Mpller-Plesset perturbation theory) correlation methods. An obvious way to treat open shells and near-degeneracies is to use multireference methods. Scientists involved in the development of such methods have often shown a strong interest in transition metal atoms and mole cules as a crucial test for their methods (1), whereas, on the other hand, transition metal chemistry is one of the domains where multireference methods have be come most popular as a computational tool (1-9).
