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A Kramers-restricted version of the closed-shell coupled cluster singles and doubles theory is presented. The theory can be used in conjunction with two- or four-component relativistic reference wave functions, where the intrinsic treatment of the spin–orbit coupling gives rise to a maximum of eight times more independent quantities (amplitudes and integrals) than found in a spin-independent formalism. The number of operations required to evaluate the equations is at most 32 times larger than in the optimal spin-independent closed-shell formalism, which is eight times smaller than found when using a spin-dependent unrestricted formalism. © 1995 John Wiley & Sons, Inc.
Visscher et al. (Sat,) studied this question.