SPIN ADAPTED RESTRICTED HARTREE-FOCK REFERENCE COUPLED-CLUSTER THEORYFOR OPEN-SHELL SYSTEMS

The coupled clusters (CC) method for effective calculations of open sh ell systems with the single restricted Hartree-Fock (ROHF) reference d eterminants is formulated. We apply the spin adaptation described in o ur previous work, aimed at removing the spin contamination in both cou pled cluster (CC) amplitudes and CC energy, both for the linear and no nlinear versions of the single and double excitation coupled cluster ( CCSD) method. We give a comparison of results with adapted and nonadap ted methods. Together with the elimination of the spin contamination, our spin adaptation yields CC equations, which reduce the number of CC amplitudes and the number of arithmetical operations, which results i n computational time comparable with analogous closed shell calculatio ns. The complication in the full spin adapted CC method is that indivi dual spin states require specific spin adapted excitations, so that fo r every spin state, a different implementation is needed. It is possib le to define a very effective approximation to the full spin adapted m odel that is common to all half-spin states. The method is less compli cated in implementation, while the computational requirements increase in comparison with the full spin adapted method only slightly. We als o suggest how to introduce spin adaptation in a very simple way to exi sting ROHF CCSD programs.