SPIN-RESTRICTED OPEN-SHELL COUPLED-CLUSTER THEORY

Spin-restricted CC theory is suggested as a new approach for the treat ment of high-spin open-shell systems in CC theory. Spin constraints ar e imposed on the wave function in the sense that the projected spin ei genvalue equations are fulfilled within the (truncated) excitation spa ce. These constraints allow a reduction in the number of independent a mplitudes, thus decreasing the computational cost when implemented eff iciently, The approach ensures that the spin expectation value always corresponds to the exact value, though the wave function is (for trunc ated CC treatments) not rigorously spin-adapted. For the specific case of high-spin doublets, detailed equations are derived for amplitudes, energy and first derivatives of the energy within the computationally most useful singles and doubles approximation. Numerical examples dem onstrate the excellent performance of the spin-restricted CC approach relative to the more traditional spin-orbital based CC treatments, sug gesting that it might be an attractive alternative for the treatment o f high-spin open-shell systems. (C) 1997 American Institute of Physics .