Quantum mechanics coupled cluster methods

The coupled cluster method is a major theory developed to efficiently accumulate electron correlations (McWeeny 1992). This method was developed by Cfzek based on the ansatz of the cluster expansion in statistical mechanics (Cfzek 1966) and became used in quantum chemistry calculations in the 1980s. In this method, the wavefunction is expanded as... 

The interactions present in halogen-bonded systems are in general weak, thus the need to use quantum mechanical methods that include electron correlation for an accurate description. In particular, these interactions are sensitive, not only to the basis set used but also to the level of electron correlation used, the counterpoise correction and the inclusion of spin-orbit effects for bromine and iodine. In contrast with the widely accepted idea that hydrogen bonded systems are reasonably well described with MoUer-Plesset perturbation (MP2) methods, halogen bond interactions are overestimated at this level. Therefore, for an accurate description of these systems a coupled cluster method is advised, preferably with single and double excitations (CCSD) or with perturbative triple excitations (CCSD(T)). This problem pertains also to density functional theory methods (DFT) where some methods lead to better results than others and the choice of the best functional is not straightforward. DFT methods have the advantage over wave function methods that time... 

As I have argued, errors are seldom computed by independent ab inito criteria in any of the calculations in theoretical chemistry which I discuss. Only the Self-Consistent Field calculations provide an upper bound whereas Many-Body Perturbation Theory and Coupled Cluster methods do not. More importantly perhaps, none of these methods computes a lower bound. As was remarked earlier the calculation of the ground state energies of atoms has been achieved to a remarkable degree of accuracy and similarly calculations on small or even medium sized molecules have given encouraging results. However, whether one can draw the conclusion that chemistry has been reduced rather depends on one s criteria of reduction. If we are to define approximate reduction as has been suggested in this paper then it must be concluded that chemistry is not even approximately reduced to quantum mechanics. The point I wish to emphasize is that we should not be misled by the apparent quantitative successes achieved and should appreciate the full nature of the approximation procedures employed. 

T. J. Lee and G. E. Scuseria, Achieving chemical accuracy with coupled-cluster theory, in S. R. Langhoff (ed.), Quantum-Mechanical Electronic Structure Calculations with Chemical Accuracy, Kluwer, 1995, p.47. P. R. Taylor, Coupled-cluster methods in quantum chemistry, in B. O. Roos (ed.). Lecture Notes in Quantum Chemistry II, Lecture Notes in Chemistry Vol. 64, Springer-Verlag, 1994, p. 125. 

Mainframes are large computers comprised of a cluster of tightly coupled machines or having multiple processors. These units will often be set up for specific applications as database servers, or for handling calculations such as those generated by quantum mechanics-based computational chemistry methods. 

Coupling of quantum mechanical molecular subsystems with larger classically treated subsystems has traditionally involved electronic structure models describing molecules embedded in a dielectric medium and this is a research area that has expanded tremendously over the last three decades [2-36]. Most of this work has involved electronic structure methods that have been based on uncorrelated electronic structure methods [2-12,15-19]. Accurate description of the electronic structure of molecular systems requires that the correlated electronic motion in the molecule is incorporated and therefore a number of correlated electronic structure methods have been developed such as the second order Moller-Plesset (MP2) [28,30,90,91], the multiconfigurational self-consistent reaction field (MCSCRF) [13,20] and the coupled-cluster self-consistent reaction field (CCSCRF) method [36]. 

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