COUPLED-CLUSTER RESPONSE FUNCTIONS REVISITED

We introduce an inherently real coupled cluster time-dependent expecta tion value of a Hermitian operator. Based on the expansion of this exp ectation value in orders of the generally time-dependent perturbation, we subsequently identify the coupled cluster time-independent expecta tion value, the linear response function, and the quadratic response f unction. The response functions and their residues behave physically c orrectly. Spectroscopic observables are identified as residues, wherea s the identification of individual transition matrix elements is prohi bited. Thus the unphysical behavior of previously published coupled cl uster response functions may be viewed not as a consequence of the pro jection, but rather that identifications are made on the basis of an u nphysical expectation value or quasienergy. (C) 1997 American Institut e of Physics.