The unitary coupled-cluster approach and molecular properties. Applications of the UCC(4) method

Abstract The evaluation of the energy and analytical energy gradient of the new unitary coupled-cluster method, UCC(4), are described, and initial numerical results for this method are reported. UCC(4) energies, molecular geometries, vibrational frequencies, and dipole moments are compared with results given by other high-level techniques such as MBPT (4), CCSDT-1, CCSDT, CISDTQ and FCI.

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