Multireference coupled‐cluster theory : the internally contracted route

Abstract

Transferring the success of the coupled‐cluster expansion for single‐determinant references to multireference cases remains a challenge. The main dilemma is a proper merge of the exponential ansatz, required for extensivity of the correlation energy, with a linear ansatz, required for an unbiased treatment of near‐degenerate state interactions. We argue that the state interaction aspect is important and that therefore the Bloch equations are the necessary starting point for all true multireference coupled‐cluster theories. Considering the aspect of spin‐adaptation and orbital invariance, we arrive at internally contracted expansions, which indeed have a number of appealing formal properties, but also incur a tremendous increase in the complexity of the resulting working equations. The most striking property of internally contracted expansions is probably that a simple transformation of the reference space turns the multistate equations into state‐specific equations without introducing further approximations. We discuss the present shortcomings and perspectives of the internally contracted multireference coupled‐cluster theory and discuss issues like the completeness of the equations, alternative expansions using normal ordering, and perspectives for large active spaces and large molecules.