Spin-restricted open-shell coupled-cluster theory

Abstract

Spin-restricted CC theory is suggested as a new approach for the treatment of high-spin open-shell systems in CC theory. Spin constraints are imposed on the wave function in the sense that the projected spin eigenvalue equations are fulfilled within the (truncated) excitation space. These constraints allow a reduction in the number of independent amplitudes, thus decreasing the computational cost when implemented efficiently. The approach ensures that the spin expectation value always corresponds to the exact value, though the wave function is (for truncated 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 demonstrate the excellent performance of the spin-restricted CC approach relative to the more traditional spin-orbital based CC treatments, suggesting that it might be an attractive alternative for the treatment of high-spin open-shell systems.