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MR CC theories

The three- and four-body clusters play an even more important role in MR CC theories. In contrast to the SR formalism, where the energy is fully determined by one- and two-body clusters, the higher-than-pair clusters enter already the effective Hamiltonian. Consequently, even with the exact one- and two-body amplitudes, we can no longer recover the exact energies [71]. Here we must also keep in mind that the excitation order of various configurations from is not uniquely defined, since a given configuration... [Pg.27]

The difficulty in constructing a genuine MR CC theory stems from the fact that the generalization of the SR CC Ansatz to the MR case is far from being unambiguous. [Pg.135]

As it is usual in the MR CC theory, we also used the wave operator in a form suggested by... [Pg.472]

Along this line, in a recent paper [37] we introduced the so-called quasiparticle-based MR CC method (QMRCC). The mathmatical structure of QMRCC is more or less the same as that of the well-known SR CC theory, i.e., the reference function is a determinant, commuting cluster operators are applied, normal-ordering and diagram techniques can be used, the method is extensive, etc. The point where the MR description appears is the application of quasiparticle slates instead of the ordinary molecular orbitals. These quasiparticles are second-quantized many-particle objects introduced by a unitary transformation which allows us to represent the reference CAS function in a determinant-like form. As it is shown in the cited paper, on one hand the QMRCC method has some advantages with respect to the closely related SR-based MR CC theory [22, 31, 34] (more... [Pg.242]

The main reason why existing MR CC methods as well as related MR MBPT cannot be considered as standard or routine methods is the fact that both theories suffer from the Intruder state problem or generally from the convergence problems. As is well known, both MR MBPT/CC theories are built on the concept of the effective Hamiltonian that acts in a relatively small model or reference space and provides us with energies of several states at the same time by diagonalization of the effective Hamiltonian. In order to warrant size-extensivity, both theories employ the complete model space formulations. Although conceptually simpler, the use of the complete model space makes the calculations rather... [Pg.76]

Another way to exploit the complementarity of Cl and CC approaches was explored earlier by Meissner et al. [10]. Instead of using Cl as a source of higher-than-pair clusters and correcting CCSD, it exploits the CC theory to correct the MR CISD results. In the spirit of an earlier work on Davidson-type corrections for SR CISD [10], Meissner et al. formulated a CCSD-based corrections for both SR [72] and MR [74] CISD. The latter was later extended to higher lying excited states [73]. [Pg.27]

We have also seen that the said complementarity of the CC and Cl approaches can be exploited in a reversed order, namely that we can rely on the MR GISD results and adjust them for their lack of dynamical correlation via the Davidson-like corrections that are based on the CC theory, as proposed by Meissner et al. [10,72-74]. We have seen that, at least for the studied DZP H4 model, either variant leads to excellent results. [Pg.39]

The need for the inclusion of higher-order effects increases with the degree of quasidegeneracy of the state considered. For this reason, much effort has been devoted to the formulation of the so-called MR MBPT [28-30]. Here, however, a number of ambiguities arises, which often limits the development of practical algorithms (c/, e.g. attempts to extend the so-called CAS-PT2 method, which is based on the complete active space self-consistent field (CAS SCF) reference, to higher than the second order). In fact, we shall see that the same problem manifests itself, even when extending the standard SR CC theory to the MR case. [Pg.119]

To overcome the failures of the CCSD(T) method in dealing with bond-breaking and open-shell species, several multireference (MR) CC methods have been developed, but none of these methods is fully satisfactory. Reviews of MR CC methods noted that even now the situation in the MRCC field is not satisfactory, since none of the MRCC methods is in a wide use [D. 1. Lyakh et al., Chem. Rev., 112,182 (2012)] and a generally accepted multireference CC theory is still lacking (A. Kohn et al., WIREs Comput. Mol. Sci. 2012, doi 10.1002/wcms.ll20). [Pg.552]

We now proceed to the other option of improving on standard CCSD via various energy corrections and focus on the very recently proposed schemes that are based on the asymmetric energy formula of CC theory (9,34), We first very briefly present the basic formalism and refer the reader to the original papers for detail (34) [see also Refs. (31-33)]. At the same time we also present yet another perturbative energy correction, this time for MR CISD. We then compare the performance of these corrections using the same DZ models of HF and N2 as in Refs. (9,34). [Pg.18]

The calculations are not all at exactly the same bond length R. The basis set is indicated after the slash in the method. R, L, C, and T are basis sets of Slater-type functions. The aug-cc-pVDZ and aug-cc-pVTZ basis sets [360] are composed of Gaussian functions. SCF stands for self-consistent-field MC, for multiconfiguration FO, for first-order Cl, for configuration interaction MR, for multireference MPn, for nth-order Mpller-Plesset perturbation theory and SDQ, for singles, doubles, and quadruples. [Pg.337]


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See also in sourсe #XX -- [ Pg.26 ]




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