Multireference Brillouin-Wigner Coupled-Cluster Theory. Single-root approach.

Recently developed Brillouin-Wigner coupled-cluster theory [I. Hubac and P. Neogrady, Phys. Rev. A 50 , 4558 (1994)] is extended to a multireference case using the Hilbert space approach. We formulate the so-called single-root (one-state or state-specific) version which deals with one state at a time while employing a multiconfigurational reference wave function. Employing the Hilbert space approach to the wave operator, we present an explicit form for cluster amplitudes in a spin-orbital form within the CCSD approximation; i.e. coupled-cluster method truncated at the single and double excitation level. The method is applied to a trapezoidal H4 model system with the use of a two-determinant reference space and the results are compared with the full configuration interaction as well as other correlated multireference techniques. The method provides a balanced description of the ground state in both quasidegenerate and nondegenerate regions and deviations from the full configuration interaction energies do not exceed 0.6 mHartree.

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