Editing Computational chemistry (section)

=== Standard CCSD and CCSD(T) method ===
{{See also|Coupled cluster}}

==== Algorithm ====
CCSD and CCSD(T) methods are advanced electronic structure techniques involving single, double, and in the case of CCSD(T), perturbative triple excitations for calculating electronic correlation effects.<ref name="Sengupta-2016">{{Cite journal |last1=Sengupta |first1=Arkajyoti |last2=Ramabhadran |first2=Raghunath O. |last3=Raghavachari |first3=Krishnan |date=2016-01-15 |title=Breaking a bottleneck: Accurate extrapolation to "gold standard" CCSD(T) energies for large open shell organic radicals at reduced computational cost |url=https://onlinelibrary.wiley.com/doi/10.1002/jcc.24050 |journal=Journal of Computational Chemistry |language=en |volume=37 |issue=2 |pages=286–295 |doi=10.1002/jcc.24050 |issn=0192-8651 |pmid=26280676 |s2cid=23011794}}</ref>

==== Complexity ====

===== CCSD =====

Scales as <math>\mathcal{O}(M^6)</math> where <math>M</math> is the number of basis functions. This intense computational demand arises from the inclusion of single and double excitations in the electron correlation calculation.<ref name="Sengupta-2016" />

===== CCSD(T) =====

With the addition of perturbative triples, the complexity increases to <math>\mathcal{O}(M^7)</math>.  This elevated complexity restricts practical usage to smaller systems, typically up to 20-25 atoms in conventional implementations.<ref name="Sengupta-2016" />
[[File:Methan 2a1.png|thumb|[[Electron density]] plot of the 2a1 molecular orbital of [[methane]] at the CCSD(T)/cc-pVQZ level. Graphic created with [[Molden]] based on correlated geometry optimization with CFOUR at the CCSD(T) level in cc-pVQZ basis.]]