Coupled Cluster Theories for Strongly Correlated Molecular Systems

The basic aspects of coupled cluster (CC) theories are reviewed from the perspective of its applicability to molecular systems with strong many-body correlation effects. In practice strong correlation refers to systems where the corresponding wavefunctions are characterized by multiconfigurational character corresponding to collective excitations from the reference function/functions. Several CC formalisms specifically designed to tackle these situations are discussed. These include single reference CC methodologies accounting for high-rank excitations and multireference CC approaches. Special attention is paid to non-iterative methods, which provide a widely accepted compromise between accuracy and numerical cost. We also discuss major theoretical and computational challenges which have to be addressed for the future developments of CC methodologies.