Abstract
New coupled-cluster methods (UCC( n)) for correlation energies and properties are developed based upon a unitary cluster ansatz and a finite-order truncation of the energy functional. These methods are shown to satisfy exactly the generalized Hellmann-Feynman theorem which facilitates the evaluation of molecular properties as derivatives of the energy. The relationship to the expectation value (XCC( n)) approach is discussed, demonstrating that UCC( n) may be obtained from XCC( n) provided that a symmetric cancellation of higher-order terms is introduced.
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