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Abstract The advantages and possibilities of a unitary coupled‐cluster (CC) theory are examined. It is shown that using a unitary parameterization of the wave function guarantees agreement between a sum‐over‐states polarization propagator and response theory calculation of properties of arbitrary order, as opposed to the case in conventional CC theory. Then, using the Zassenhaus expansion for noncommuting exponential operators, explicit diagrams for an extensive and variational method based on unitary CC theory are derived. Possible extensions to the approximations developed are discussed as well. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006
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