Frequency-dependent hyperpolarizabilities in the brueckner coupled-cluster theory

A formulation for calculating frequency‐dependent hyperpolarizabilities in the coupled‐cluster (CC) theory with the Brueckner orbitals is presented. Our quasi‐energy derivative (QED) method is applied, and explicit expressions for response properties up to third order [μ, α(−ω1;ω1), β(−ωσ;ω1, ω2)] are derived. The position of the resonances is determined from the RPA‐like equation. It is found that the pole structures are not compatible with ones of the exact response functions. An extension of the coupled Brillouin–Brueckner CEPA‐0 by Kutzelnigg to the time‐dependent case is also discussed. © 1994 John Wiley & Sons, Inc.