Coupled cluster single double triple CCSD

In Table 6.3, the values of De for RfCU are compared with those obtained within various approximations using relativistic effective core potentials (RECP) Kramers-restricted Hartree-Fock (KRHF) (Han et al 1999), averaged RECP including second-order M0ller-Plesset perturbation theory (AREP-MP2) for the correlation part (Han et al. 1999), RECP coupled-cluster single double (triple) [CCSD(T)] excitations (Han et al. 1999), and a Dirac-Fock-Breit (DFB) method (Malli and Styszynski 1998). The AREP-MP2 calculation of De gives 20.4 eV, while the RECP-CCSD(T) method with correlation leads to 18.8 eV. Our value of De of 19.5 eV is just between these calculated values. 

B-CC = Brueckner CC CCD = coupled-cluster doubles CCSD = coupled-cluster singles and doubles CCSDT = coupled-cluster singles, doubles, and triples CCSDTQ = coupled-cluster singles, doubles, triples, and quadruples Cl = configuration interaction EOM = equation-on-motion FCI = full-configuration interaction. 

Unlike the CCSD model, the CCSDT and higher models - such as the coupled-cluster singles-doubles-triples-and-quadruples (CCSDTQ) model [8] - are therefore not treated in detail in this chapter. Fortunately, accurate coupled-cluster models have been developed that include the effects of the connected triples in an approximate fashion. We defer the discussion of such approximate CCSDT treatments to Chapter 14. 

Becke s three parameters Lee, Yang, Parr functional Complete Active Space Perturbation Theory 2nd Order Complete Active Space SCF Coupled Cluster Single Double CCSD (Triple)... 

CCSD(T) Coupled Cluster singles, doubles and perturbative triple excitations... 

There are cases where neither MC-GIAO nor MP2-GIAO is good enough and where more sophisticated treatments of electron correlation must be used. The climax on these lines is CCSD(T)-GIAO (where CCSD(T) = coupled cluster, singles-doubles-approximate triples). The computational cost of these sophisticated treatments is so high that it will probably remain limited to special cases. 

However, until today no systematic comparison of methods based on MpUer-Plesset perturbation (MP) and Coupled Cluster theory, the SOPPA or multiconfigurational linear response theory has been presented. The present study is a first attempt to remedy this situation. Calculations of the rotational g factor of HF, H2O, NH3 and CH4 were carried out at the level of Hartree-Fock (SCF) and multiconfigurational Hartree-Fock (MCSCF) linear response theory, the SOPPA and SOPPA(CCSD) [40], MpUer-Plesset perturbation theory to second (MP2), third (MP3) and fourth order without the triples contributions (MP4SDQ) and finally coupled cluster singles and doubles theory. The same basis sets and geometries were employed in all calculations for a given molecule. The results obtained with the different methods are therefore for the first time direct comparable and consistent conclusions about the performance of the different methods can be made. 

Valeev, E.F., Crawford, T.D. Simple coupled-cluster singles and doubles method with perturbative inclusion of triples and explicitly correlated geminals the CCSD(T)ri2 model. J. Chem. Phys. 2008, 128, 244113. 

CCSD (T) Model. Coupled Cluster Singles and Doubles with Triples correction. A Correlated Model. 

Instead of the very demanding CCSDT calculations one often performs CCSD (T) (note the parentheses), in which the contribution of triple excitations is represented in an approximate way (not refined iteratively) this could be called coupled cluster approximate (or perturbative) triples. The quadratic configuration method (QCI) is very similar to the CC method. The most accurate implementation of this in common use is QCISD(T) (quadratic Cl singles, doubles, triples, with triple excitations treated in an approximate, non-iterative way). The CC method, which is usually only moderately slower than QCI (Table 5.6), is apparently better [102]. CCSD(T) calculations are, generally speaking, the current benchmark for practical molecular calculations on molecules of up to moderate size. 

G.E. Scuseria, The open-shell restricted Hartree-Fock singles and doubles coupled-cluster method including triple excitations CCSD (T) Application to Ca+. Chem. Phys. Lett. 176, 27-35 (1991)... 

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