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An approximation scheme for dynamic response functions within the coupled cluster or exp (T) method of Coester and K\"ummel is described in this paper. Particular attention is given to the relationship between the "vacuum amplitude" 〈0|U (t, -) |0〉 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 〈₀|U (t, -) |0〉 for the time evolution into an independent-particle reference state |₀〉, which is not orthogonal to |0〉. The latter amplitude plays a prominent role in the time-dependent coupled-cluster calculation of response functions.
Dalgaard et al. (Thu,) studied this question.
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