An approximation scheme for dynamic response functions within the coupled cluster or $\exp \mathrm{}\left(T\right)\mathrm{}$ method of Coester and Kümmel is described in this paper. Particular attention is given to the relationship between the "vacuum amplitude" $〈0\left|U\right(t,-\infty \left)\right|0\mathrm{}〉$ for the time development of an exact eigenstate of a many-electron Hamiltonian in the presence of an adiabatically switched-on perturbaction, and the amplitude $〈{\Phi }_0\left|U\right(t,-\infty \left)\right|0\mathrm{}〉$ for the time evolution into an independent-particle reference state $|{\Phi }_0〉$, which is not orthogonal to $|0\mathrm{}〉$. The latter amplitude plays a prominent role in the time-dependent coupled-cluster calculation of response functions.

• Received 25 February 1983

DOI:https://doi.org/10.1103/PhysRevA.28.1217