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Abstract
The coupled-clusters singles and doubles (CCSD) + T(CCSD) approach was proposed 25 years ago together with a suggestion of a coupled cluster hierarchy including triply excited configurations, known as CCSDT-n. This retrospective recapitulates the essence of those approaches in the contemporary context. Finally, the relation to other developments in this field is briefly outlined with emphasis on work performed at the Quantum Theory Project.
Acknowledgements
Since the first scientific stay of one of us (MU) at QTP in 1984 we visited QTP repeatedly in years 1984–1993 for a longer or shorter periods of time. We appreciate very much the unforgettable, creative and friendly social and scientific atmosphere at QTP. We particularly wish to thank Rod Bartlett for his wonderful hospitality and for our fruitful collaboration throughout many years. Currently, the work of the authors in this field has been supported by the Grant Agency of the Ministry of Education of the Slovak Republic and Slovak Academy of Sciences (VEGA project Nos 2/0079/09 and 1/0520/10) and also benefits from the Centers of Excellence program of the Slovak Academy of Sciences (COMCHEM, Contract no. II/1/2007).