各位前辈，我在JCP的一篇文献中看到这么一段（文献title：Electronic structure of NSO− and SNO− anions: Stability, electron affinity, and spectroscopic properties），For the evaluation of ground-state and lowest excited states accurate equilibrium geometric parameters, rotational constants, and harmonic vibrational frequencies, the coupled-cluster singles and doubles model with perturbative contributions of connected triple excitations ((R)CCSD(T)) was chosen, in the suitable formalism for closed-shell or open-shell systems, see Ref. 21and references therein.为什么此处的(R)CCSD(T)可以用来计算开壳层体系，表示有些疑惑，麻烦老师解答一下，谢谢！ps：参考文献21比较了CCSD与其他一些算法的优越性。