In fact the Ω C (ω) operator are the solution of the same equation which appears in the time-dependent coupled-clusters theory of Monkhorst 25,26 and represents the response of the system to the external perturbation given by the operator X. The S C operator in the Eq. ( ) has been introduced by Jeziorski and Moszynski 27 in order to provide an explicitly connected expression for the expectation values in the coupled cluster theory. A formal definition of the S C cluster operator in terms of CC amplitudes T C reads: e