Coupled-cluster CCSD method

Besler, B. H., Scuseria, G. E., Scheiner, A. C., and Schaefer, FI. F., A systematic theoretical study of harmonic vibrational frequencies The single and double excitation coupled cluster (CCSD) method, J. Chem. Phys, 89, 360-366 (1988). 

B. H. Besler, G. E. Scuseria, A. C. Scheiner, and H. E Schaefer III,/. Chem. Phys., 89,. 350 (1988). A Systematic Theoretical Study of Harmonic Vibrational Frequencies The Single and Double Excitation Coupled Cluster (CCSD) Method. 

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.

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. 

J. R. Thomas, B. J. DeLeeuw, G. Vacek, and H. F. Schaefer,/. Chem. Phys., 98,1336 (1993). A Systematic Study of the Harmonic Vibrational Frequencies for Polyatomic Molecules The Single, Double, and Perturbative Triple Excitation Coupled-Cluster [CCSD(T)] Method. 

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. 

The Coupled-clusters (CC) method[7] based on the cluster expansion of the wavefunction has been established as a highly reliable method for calculations of ground state properties of small molecules with the spectroscopic accuracy. When this method is used together with a flexible basis set it recovers the dominant part of the electron correlation. Typically, CC variant explicitly considering single and double excitations (CCSD) is used. In order to save computer time the contributions from triple excitations are often calculated at the perturbation theory level (notation CCSD(T) is used in this case). CCSD(T) method can be routinely used only for systems with about 10 atoms at present. Therefore, it cannot be used directly in zeolite modeling, however, results obtained at CCSD(T) level for small model systems can serve as an important benchmark when discussing the reliability of more approximate methods. 

A Systematic Study of the Harmonic Vibrational Frequencies for Polyatomic Molecules. The Single, Double, and Perturbative Triple Excitation Coupled-Cluster [CCSD(T)] Method. 

All of the methods discussed above are based on a multi-reference (MR) approach to obtaining wavefunctions and properties. Such MR approaches are often necessary, because many chemical problems involve species with considerable configurational mixing due the existence of near degeneracies (diradical character). However, the amount of diradical character in a chemical system can span a very broad range, from essentially zero (e.g. HOMO occupancy 2, LUMO occupancy 0) to fully diradical (HOMO occupancy 1, LUMO occupancy 1). As one approaches fully diradical character, all single reference methods break down, but they do not break down at the same rate as one approaches this limit. In particular, there is considerable evidence that coupled cluster (CC) methods, particularly those like CCSD(T) that incorporate a triples correction, can overcome the deficiency of a single reference wavefunction for problems with non-trivial diradical character. This has been demonstrated, for example, by... 

The improvements within the hierarchy of A -electron models is probably a more complicated task. There are a couple of quantum-chemistry theories that allow us to approach the exact Schrodinger equation systematically. Among them the coupled-cluster (CC) method represents probably the most successful approach. It can be applied to relatively large systems and the theory is both size-extensive and size-consistent. So far the only way to approach the exact Schrodinger equation, within the hierarchy of the A -electron models (following the horizontal axis on Figure 1), is the systematic extension of the excitation level. In CC theory there is a series of models that refer to the way the cluster operator is truncated (CCS, CCSD, CCSDT, CCSDTQ and so on). In the limit of the untruncated cluster operator the CC wave function becomes equivalent with full Cl, which is the exact solution of the Schrodinger equation within a particular basis set. The truncation level indicates, in some sense, the accuracy of the model which is almost always limited by the available computational resources. 

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