Abstract
Unitary-group-based perturbation theory for both low and high spin open-shell systems is employed to obtain perturbative corrections for the contribution of the connected triexcited clusters as well as of pseudo-doubles (three-body doubles), within the framework of the unitary-group-based coupled-cluster method with interacting singles and doubles [UGA-CCSD(is)]. A set of triple excitation operators, generating orthonormal triply excited configurations, which can be employed in both noniterative and iterative coupled–cluster calculations involving connected triexcited clusters, is constructed and various schemes for perturbative triple corrections are introduced. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 65–75, 1998
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