Abstract
The role of the connected quadruple excitations in the coupled-cluster (CC) theory is discussed. The full inclusion of the T 4 (Q) operator in addition to singles (S), doubles (D) and triples (T) defines the CCSDTQ method which offers a very accurate computational tool applicable to small molecular systems. The efficient organization of the CC equations results in the quasilinear formulation of the CCSDTQ scheme. A wider range of applications can be ensured with the approximate variants of the CCSDTQ approach. Due to possible factorization of the lowest order quadruple contribution to the energy, a noniterative scheme has been formulated which requires n 7 scaling. Performance of the CCSDTQ method has been discussed on the basis of the results obtained for several small molecules in confrontation with the reference full configuration interaction data.
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