Coupled Cluster - Types of Coupled-cluster Methods

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

  1. S - for single excitations (shortened to singles in coupled-cluster terminology)
  2. D - for double excitations (doubles)
  3. T - for triple excitations (triples)
  4. 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:

  1. A coupled-cluster method
  2. It includes singles and doubles fully
  3. Triples are calculated non-iteratively.

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