Coupled cluster response functions revisited

We introduce an inherently real coupled cluster time-dependent expectation value of a Hermitian operator. Based on the expansion of this expectation value in orders of the generally time-dependent perturbation, we subsequently identify the coupled cluster time-independent expectation value, the linear response function, and the quadratic response function. The response functions and their residues behave physically correctly. Spectroscopic observables are identified as residues, whereas the identification of individual transition matrix elements is prohibited. Thus the unphysical behavior of previously published coupled cluster response functions may be viewed not as a consequence of the projection, but rather that identifications are made on the basis of an unphysical expectation value or quasienergy.