Editing Coupled cluster (section)

== Types of coupled-cluster methods ==

The classification of traditional coupled-cluster methods rests on the highest number of excitations allowed in the definition of <math>T</math>. 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 <math>T</math> operator in CCSDT has the form

: <math>T = T_1 + T_2 + T_3.</math>

Terms in round brackets indicate that these terms are calculated based on [[perturbation theory]]. For example, the CCSD(T) method means:
# Coupled cluster with a full treatment singles and doubles.
# An estimate to the connected triples contribution is calculated non-iteratively using [[Perturbation theory (quantum mechanics)|many-body perturbation theory]] arguments.