Why CCSD(T) works: a different perspective

The CCSD(T) method was originally motivated as an attempt to treat the effects of triply excited determinants upon both single and double excitation operators on an equal footing. Hence, conventional analyses based on perturbation theory cannot satisfactorily explain why the particular fifth-order term included in CCSD(T) should be chosen over a number of other possibilities. This work demonstrates that the terms appearing in CCSD(T) can be justified if one takes the biorthogonal representation of the CCSD state as the zeroth-order wavefunction. This perspective provides some additional insight as to why the method works so well in practice. (C) 1997 Elsevier Science B.V.