Spin-restricted Brueckner orbitals for coupled-cluster wavefunctions

The theory of spin-restricted Brueckner orbitals for high-spin open-shell coupled-cluster wavefunctions is presented. The orbitals are based on single-excitation amplitudes constructed using the symmetric spin–orbital basis coupled-cluster method of Jayatilaka and Lee. It is shown how this approach may be easily implemented within existing open-shell coupled-cluster programs. The method’s performance is compared to conventional spin-unrestricted Brueckner orbitals for C̃ 2A2 NO2 and X̃ 2A2′ NO3, for which instabilities in the Hartree–Fock reference determinant cause serious difficulties for highly correlated wavefunctions.