SPIN-RESTRICTED BRUECKNER ORBITALS FOR COUPLED-CLUSTER WAVE-FUNCTIONS

The theory of spin-restricted Brueckner orbitals for high-spin open-sh ell 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 show n how this approach may be easily implemented within existing open-she ll coupled-cluster programs. The method's performance is compared to c onventional spin-unrestricted Brueckner orbitals for (C) over tilde(2) A(2)NO(2) and (X) over tilde(2)A(2)'NO3, for which instabilities in th e Hartree-Fock reference determinant cause serious difficulties for hi ghly correlated wavefunctions. (C) 1997 American Institute of Physics. [S0021-9606(97)03747-1].