Types of Coupled-cluster Methods
The classification of traditional coupled-cluster methods rests on the highest number of excitations allowed in the definition of . The abbreviations for coupled-cluster methods usually begin with the letters "CC" (for coupled cluster) followed by
- S - for single excitations (shortened to singles in coupled-cluster terminology)
- D - for double excitations (doubles)
- T - for triple excitations (triples)
- Q - for quadruple excitations (quadruples)
Thus, the operator in CCSDT has the form
Terms in round brackets indicate that these terms are calculated based on perturbation theory. For example, a CCSD(T) approach simply means:
- A coupled-cluster method
- It includes singles and doubles fully
- Triples are calculated non-iteratively.
Read more about this topic: Coupled Cluster
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