Optimized-orbital coupled-cluster effect

Before proceeding to open-shell theory, it is worth noting that CCSD properly treats the nondynamical effects that are missing in a single-determinant reference function, which were discussed in section 12.1. This is because the coupled-cluster wave function is an infinite-order expansion to the given excitation level the coefficients of the determinants that complete the reference expansion and all the excitations from these are included and optimized in the coupled-cluster wave function. Also, the presence of single excitations accounts for the orbital relaxation that would correct the distortion of the reference determinant. 

---