Abstract
Second derivatives of the energy correspond to second-order response properties and molecular force constants. Currently, both the theory and application of analytic second derivatives in many-body methods are limited to second-order perturbation theory. The general theory of analytic second derivatives for the coupled-cluster (CC) model is presented. The analytic expressions for the second derivative of the energy are given in terms of the response (or ‘‘relaxed’’) density, discussed in part I, and the first-derivative t amplitudes for efficient evaluation. Explicit expressions for the second derivatives of the coupled-cluster singles, doubles, and linearized triples model (CCSDT-1) are presented. Analytic derivatives for the finite-order MBPT(3) and MBPT(4) models are derived as special cases of the theory.
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