A multireference coupled-cluster method for special classes of incomplete model spaces

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Abstract
A multireference coupled-cluster (MRCC) method for special but general classes of incomplete model spaces is developed within a Hilbert space framework. The formulation avoids the valence universality requirement and related Fock space considerations that require hierarchical solutions for different number of electrons. Consequently, the Hilbert space approach has fewer amplitudes to determine. It is shown that diagonalization of the effective Hamiltonian leads to purely extensive energies, providing the formal basis for a general MR-CC methods for potential energy surfaces.