Single bonds breaking

This reaction proceeds by a concerted, [3,3] sigmatropic rearrangement (cf. the Claisen rearrangement) where one carbon-carbon single bond breaks, while the new one is formed. It is a reversible reaction the thermodynamically more stable isomer is formed preferentially ... 

The most popular use of the UHF solutions concerned the single bond breaking, since it was rapidly understood that while the RHF solution of H2... 

The motivation behind the LMMCC approximation stems from the success of the CR-CCSD[T] and CR-CCSD(T) methods in describing single bond breaking... 

Therefore, the SF ansatz (2) is sufficiently flexible to describe changes in ground state wavefunctions along a single bond-breaking coordinate. Moreover, it treats both closed-shell (e.g., N and Z) and open-shell (V and T) diradicals states in a balanced fashion, i.e., without overemphasizing the importance of one of the configurations. 

The first conclusion we may draw is that the minimal dressing of a Cl matrix (CAS-SDCI in our study) not only ensures the importanty property of size-extensivity, but also improves the absolute values of the yielded energies. This can be stated in all studied cases single bond breaking, two single bond breaking and triple bond breaking. 

An interesting alternative to the externally corrected MMCC methods, discussed in Section 3.1.1, is offered by the CR-EOMCCSD(T) approach [49, 51,52,59]. The CR-EOMCCSD(T) method can be viewed as an extension of the ground-state CR-CCSD(T) approach of Refs. [61,62], which overcomes the failures of the standard CCSD(T) approximations when diradicals [76,104,105] and potential energy surfaces involving single bond breaking and single bond insertion [49,50,52,60-62,65,67,69,70,72,73] are examined, to excited states. 

QCISD(T) method. However our own experience is that at least for single bond breaking the QCISD(T) method provides a satisfactory solution out to much greater bond separations than does the CCSD(T) method. Our unpublished sample comparisons of heats of formation for a wide range of pure hydrocarbon and oxygenated hydrocarbons suggest little difference otherwise. For this reason, we prefer the QCISD(T) method. 

In this chapter, we have reviewed our recent effort toward the extension of the linear scaling local correlation approach of Li and coworkers [38 0], abbreviated as CIM, to the standard CCSD approach and two CC methods with a non-iterative treatment of connected triply excited clusters, including the conventional CCSD(T) method and its completely renormalized CR-CC(2,3) analog [102] (see, also, W. Li and P. Piecuch, unpublished work). The local correlation formulation of the latter method based on the CIM formalism is particularly useful, since it enables one to obtain an accurate description of single bond breaking and biradicals, where CCSD(T) fails, with an ease of a black-box calculation of the CCSD(T) type [24-26, 109-117]. At the same time, CR-CC(2,3) is as accurate as CCSD(T) in applications involving closed-shell molecules near their equilibrium geometries. 

Although calculations of entire molecular PESs involving single bond breaking require using CR-CCSD[T] and CR-CCSD(T) methods rather than their simplified renormalized versions [11-13,30,31,33,35,37], these R-CCSD[T] and R-CCSD(T) approaches allow us to understand the relationship between the standard and completely renormalized CC approaches. 

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