Can you explain the difference between these two computational methods ?
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asked Nov 29, 2011 at 20:50
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2 Answers
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In short, (T) means a perturbative (non-iterative) correction to a CCSD calculation. From a talk demonstrating the derivation of the CCSD(T) method:
The CCSD(T) method consists of a perturbative (noniterative) correction to the CCSD energy.
• Utilizing the expressions from perturbation theory, approximate second order triples amplitudes *$\hat{T}^{(2)}_3$ are generated from the CCSD $\hat{T}_2$ amplitudes (rather than from first order amplitudes).
• The second order corrected wave function is then used to compute the fourth and fifth order energy corrections, which are added to the CCSD energy.
Wikipedia has a decent page about CC in general. A very good review[1] by T. Daniel Crawford and Henry F. Schaefer III is available online. There is a section discussing the (T) correction specifically. Rev. Comput. Chem. 2007, 33–136
References:
- Crawford, T. D.; Schaefer, H. F. An Introduction to Coupled Cluster Theory for Computational Chemists. Rev. Comput. Chem. 2007, 33–136. DOI: 10.1002/9780470125915.ch2.
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$\begingroup$ Yann: Your links to UGA are broken now. $\endgroup$ Apr 13, 2013 at 20:17
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There are many ways to answer this, but CCSD is only complete through third-order in many-body perturbation theoretic terms, while CCSD(T) is complete through fourth-order. It was the desire to have a fourth-order complete method that led Bartlett and coworkers to develop CCSD[T]. The CCSD(T) method includes a fifth-order term (at least in the RHF case), which was later shown to be fourth-order in the ROHF case by Stanton, which helps to explain why the inclusion of that term has such an impact on the accuracy of CCSD(T) relative to CCSD[T].
The CFOUR Bibliography page has a list of most (all?) the seminal papers on coupled-cluster theory, which might be of interest.
