Multi‐Reference Configuration Interaction

Abstract

The multireference configuration interaction (MRCI) method is a simple and robust way of treating non-dynamic and dynamic correlation on an equal footing. In its original form, it is based on a straightforward linear expansion of the wave function into Slater determinants or configuration state functions (CSFs) and the application of the variational principle. In this chapter the construction of this expansion space by means of reference configurations and a hierarchy of excitations thereof is discussed and a classification of the corresponding orbital spaces is provided. The violation of size-extensivity is explained using a simple model example and the possibilities of a posteriori and intrinsic corrections to restore approximate size-extensivity are presented. Contraction schemes based on internally contracted (ic)-MRCI, which alleviate the high computational demand of the uncontracted MRCI, are explained. The description of configuration selection schemes and different options for choosing orbitals to be used in the MRCI conclude this chapter.