| The new single-reference approach to the many-electron correlation problem, termed the method of moments of coupled-cluster equations (MMCC), which can be used to extend the applicability of the standard CC methods to bond breaking, is further developed by considering the higher-than-quadruply excited moments of the CCSD (coupled-cluster singles and doubles) equations. The main idea behind the MMCC theory is that of the noniterative a posteriori energy corrections δ which, when added to the energies obtained in the standard CC calculations, recover the exact, full configuration interaction (CI), energies. In this study, the MMCC theory is extended by incorporating the generalized moments of the CCSD equations that correspond to projections of these equations on pentuply and hextuply excited configurations into the MMCC energy corrections. It is demonstrated that these higher-order moments can be very important in studies of the most difficult cases of multiple bond breaking. The trial wave functions that are used to calculate the MMCC energy corrections are provided by the relatively inexpensive CI methods and by the truncated forms of the exponential, CC-like, wave functions. The resulting CI-corrected MMCC methods and the quadratic MMCC (QMMCC) methods including higher-than-quadruply excited moments of the{09}CCSD equations are tested in studies of different types of bond breaking, including the single bond breaking in the HF molecule, the simultaneous breaking of both O–H bonds in the water molecule, the complicated case of triple bond breaking in the nitrogen molecule, and the extremely difficult case of bond breaking in the C 2 molecule. |
