Proposal of multi-root multi-reference coupled cluster formalisms

This work first returns on the intrinsic difficulties of multi-reference coupled cluster (MR-CC) formalisms. They may be formulated either in an effective Hamiltonian frame or in an intermediate effective Hamiltonian (IEH) one. In the former case complete model space approach being intractable, the incomplete model space approach is re-examined, and is formulated in terms of an IEH, despite the fact that the model space dimension is equal to the number of desired roots. Some of its drawbacks are illustrated on the magnetic systems problem. Then one proposes a multi-root complete active space (CAS)-based CC-SD, which only handles single and double excitation operators, generalising a previously proposed State Specific MR-CC formalism. The method proceeds through an iterative dressing of the matrix elements between the singles and doubles and the CAS determinants.