I just finished reading a new article by Musial, Perera, and Bartlett on multireference coupled cluster theory. This is an interesting paper that considers a simple but ingenious MRCC approach based on the equation of motion formalism.

For a CAS(2,2) problem, the reference wave function is taken to be a closed shell determinant with the active orbitals either fully occupied or empty. One then solves for the coupled-cluster wave function for this double anion state to get an intermediate wave function . The MR ansatz used by Musial *et al*. is , where removes a pair of electrons among all the occupied orbitals in

while is an operator that relaxes the wave function via ordinary excitations truncated at a give excitation level

As the authors claim, the theory is simple, and simple in a multireference many-body approach is good. However, there is one problem with this approach. The energy is very sensitive to the choice of the original orbitals. In my experience, state-specific approaches based on the Jeziorski-Monkhorst ansatz tend to be a bit less sensitive. It would be great if the theory could also determine some optimal set of MOs (Bruckner, NOs, …).

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