Coupled-cluster CCSD and

C. Hampel, K. A. Peterson, and H.-J. Werner, Chem. Phys. Lett., 190, 1 (1992). A Comparison of the Efficiency and Accuracy of the Quadratic Configuration Interaction (QCISD), Coupled-Cluster (CCSD), and Brueckner Coupled-Cluster (BCCD) Methods. 

Table4.4 Spectroscopic properties for Au2 q= -1,0, + 1) using ab-initio (Hartree Fock, HF, second-order Moller-Plesset, MP2, and coupled cluster, CCSD(T)) and DFT (local spin-density approximation, LSDA, Perdew-Wang CCA, PW91, and Becke three-parameter Lee-Yang-Parr functional, B3LYP) methods at the RPPA level of theory.

The computational complexity of the coupled-cluster method truncated after a given excitation level m - for example, m = 2 for CCSD - may be discussed in terms of the number of amplitudes (Nam) in the coupled-cluster operator and the number of operations (Nop) required for optimization of the wavefunction. Considering K atoms, each with Nbas basis functions, we have the following scaling relations ... 

In the SOPPA(CCSD) method [36] the Mqller-Plesset correlation coefficients and are replaced in all SOPPA matrix elements by the corresponding coupled cluster singles and doubles amplitudes and r , whereas in the earlier CCSDPPA method [52,53] only some of the Mqller-Plesset correlation coefficients were replaced. Although SOPPA(CCSD) is based on a CCSD wavefunction, it is still only correct through second order and not the linear... 

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. 

S. R. Gwaltney and M. Head-Gordon, A second-order perturbation correction to the coupled-cluster singles and doubles method CCSD(2). J. Chem. Phys. 115, 2014 (2001). 

Using coupled cluster CCSD(T)//B3P86 methods, the Ea values for HERON rearrangement of model hydrazines 215b-d have also been computed to be between 24 and 34 kcalmoU The authors ruled out alternative concerted rearrangements, which were computed to be energetically unfavourable. 

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. 

Kohn, A., Richings, G.W., Tew, D.P. Implementation of the full explicitly correlated coupled-cluster singles and doubles model CCSD-F12 with optimally reduced auxiliary basis dependence. J. Chem. Phys. 2008, 129, 201103. 

CCSD Model. Coupled Cluster Singles and Doubles. A Correlated Model. 

The single-reference coupled cluster (CC) theory [1-5] has become a standard computational tool for studying ground-state molecular properties [6-10]. The basic approximations, such as CCSD (coupled cluster singles and doubles approach) [11-15], and the noniterative CCSD[T] [16,17] and CCSD(T) [18] methods, in which the cleverly designed corrections due to... 

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