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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.
Acknowledgements
It is a great pleasure to contribute this paper to a special volume celebrating the 50th Sanibel Symposium and work done in the Quantum Theory Project in Gainesville. We would like to express our gratitude for the hospitality of Professor Rodney J. Bartlett, faculty members and staff during our visits in QTP.
Current positions: Professor of Chemistry (SAK), Associate Professor (MM), Institute of Chemistry, University of Silesia, Katowice, Poland.
Quantum Theory Project: SAK – Postdoctoral Associate (1982–1984, 1988–1989), several 2–6 months visits in the years 1985–2002; MM – Postdoctoral Associate (2002–2003), several 2–4 months visits in the years 2004–2010.