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
.