Couple cluster methods advantages

MBPT starts with the partition of the Hamiltonian into H = H0 + V. The basic idea is to use the known eigenstates of H0 as the starting point to find the eigenstates of H. The most advanced solutions to this problem, such as the coupled-cluster method, are iterative well-defined classes of contributions are iterated until convergence, meaning that the perturbation is treated to all orders. Iterative MBPT methods have many advantages. First, they are economical and still capable of high accuracy. Only a few selected states are treated and the size of a calculation scales thus modestly with the basis set used to carry out the perturbation expansion. Radial basis sets that are complete in some discretized space can be used [112, 120, 121], and the basis... 

A major advantage of the intermediate Hamiltonian approach is the flexibility in selecting the model space. This has been a major problem in applying the Fock-space scheme, as described at the beginning of this section. While in the Fock-space coupled cluster method one may feel lucky to find any partitioning of the function space into P and Q with convergent CC iterations, the intermediate Hamiltonian method makes it possible for the first time to vary the model space systematically and study the effect upon calculated properties. An example is given in Table 3, which shows the dependence of the calculated electron affinity of Cs on the model spaces Pm and Pi [55]. 

Recently Lindroth (30) has introduced the concept of complex coordinates into nonvariational methods. The idea of Lindroth is to produce a basis set of one-particle complex rotated functions as solutions of the one-particle complex-rotated problem, and then to apply this basis within a bound-state method for a many-particle system. In this way the complex rotation has been combined with the many-body perturbation theory (30) and with the coupled cluster method (31). Apart from the fact that the resulting energy is complex and its real and imaginary part can be interpreted as the position and width of a resonance under consideration, the use of complex coordinates has the advantage that singularities caused by the degeneracy on the real axis between the resonance and the adjacent continuum are not present in the complex energy plane. 

The coupled cluster method is the Cl method based on this wavefunction. One of the noain advantages of this method is to ensure size consistency (see Sect. 3.3) in the selection of electron configurations. In coupled cluster methods, the coupled-cluster singles and doubles (CCSD) method terminates f up to the double excitation... 

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... 

The accuracy of these two methods is very similar. The advantage of doing coupled cluster calculations is that it is a size extensive method (see chapter 26). Often, coupled-cluster results are a bit more accurate than the equivalent... 

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