Current status of the multiconfiguration–configuration interaction (MC–CI) method as applied to molecules containing transition-metal atoms

Abstract

The current status of the MC–CI method is reviewed. Calculations on transition metals show that the MCSCF step is often the most problematic part of these types of calculations. The reason is that the most desirable MCSCF calculation can easily be too large for presently available methods. The limitation lies in the number of configurations which can be handled. A new direct CASSCF CI method is suggested which is capable of efficiently handling a larger number of configurations. In the multi-reference CI step, the limitation is often in the size of the reference space rather than the total number of configurations. Methods are therefore needed in which the internal space is more efficiently handled, particularly with respect to the number of formulae which need to be stored. The new CASSCF CI method can also be useful in this context.