Coupled cluster with singles and doubles

The CC2 method [74] is an approximation to coupled cluster with singles and doubles (CCSD), and the excited state energies calculated have MP2 quality. An implementation that employs the resolution of identity (RI) approximation for two-electron integrals to reduce the CPU time is also available, RI-CC2 [75], which is suitable for large scale integral-direct calculations. This method has been implemented in TURBOMOLE [76],... 

CCSD Coupled cluster with single and double substitution operators... 

CCSD(T) coupled cluster with single and double excitations (followed by a perturbation treatment of triple excitations)... 

CCSD(T) Coupled cluster with singles and doubles (+ perturbative triplets)... 

CCSD Coupled-cluster with singles and doubles... 

Coupled-cluster with single and double excitation cluster operators Coupled-cluster with single, double, and triple excitation cluster operators... 

WFT treatment such as Moeller-Plesset second-order perturbation theory (MP2) or coupled-cluster with single and double excitations (CCSD(T)) with correlation energy [51]. This approximation works fairly well for large interfragment distances but is obscured for shorter bonds by exchange and intrafragment correlation effects. 

Two of the most commonly used electronic structure methods to evaluate Eint are MP2 (second-order MoUer-Plesset perturbation theory) and CCSD(T) (coupled-cluster with singles and doubles and non-iterated triples). Interaction energies calculated using these methods are shown in O Table 6-1. Interaction energies calculated using MP2 are usually close to those calculated using the more sophisticated and computationally expensive CCSD(T), but the exception is the benzene dimer system for which MP2 overestimates the binding by almost a factor of 2. This deserves some explanation. 

Coupled cluster with singles and doubles excitations (CCSD) is a size-consistent post-HF electron correlation method. The wavefunction, Y, in coupled cluster theory is formulated in terms of a cluster (exponential) expansion including the single and double excitation operators 7i and %. The effect of triple excitations (T) is calculated with perturbation theory. 

CCSD = coupled-cluster with singles and doubles CCSD(T) = CCSD with noniterative triple excitations Cl = configuration interaction CISD = Cl singles and doubles CP = counterpoise correction QCISD = quadratic Cl with singles and doubles. 

The methods of coupled clusters with single- and double-excitation terms (CCSD), CCSD with noniterative triples [CCSD(T)], and the closely related approximate method of quadratic Cl (QCISD) and QCISD with noniterative triples [QCISD(T)] have been used to study a few hydrogen-bonded complexes. These methods are size-consistent, infinite-order methods, and as such are deemed to be most reliable. Unfortunately, CCSD(T) is computationally the most expensive method, and is not feasible for routine use on larger systems. CCSD(T) is, however, the correlation method which should be used for benchmarking purposes in small complexes, and the method of choice for difficult cases. 

MR-MBPT multi-reference many-body perturbation theory MR-MPPT multi-reference Mpller-Plesset perturbation theory CCSD single-reference coupled cluster with single and double replacements... 

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