CR-CCSD

It has been demonstrated in several benchmark calculations that the CR-CCSD(T) (completely renormalized CCSD(T)) and CR-CCSD(TQ) (completely renormalized CCSD(TQ)) methods provide an excellent description of entire PESs involving single and double bond dissociation (P, 13, 15, 17-19, 21, 111), highly-excited vibrational term values near dissociation 17, 18, 21, 111), and... 

Our recent numerical experiments with the Cl-corrected MMCC methods indicate that in looking for the extensions of the CR-CCSD[T], CRCCSD(T), and CR-CCSIXfQ) methods that would provide an accurate description of triple bond breaking one may have to consider the approximations that use the pentuply and hextuply excited moments of the CCSD equations, M (2), k = 5 and 6, respectively (21). The CR-CCSD[T] and CR-CCSD(T) methods use only the triexcited CCSD moments M (2), whereas the CR-CCSD(TQ) approaches use the tri- and tetraexcited moments, (2) and (2),... 

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... 

In consequence, the LMMCC energy equals the CR-CCSD[T] energy, eq... 

These terms are present in the wave function T(,) defining variant b of the CR-CCSD(TQ) theory (the CRCCSD(TQ),b approach of ref 14). In consequence, the CR-CCSD(TQ),b results for multiple bond breaking are considerably better than the CRCCSD[T] and CR-CCSD(T) results (14). The Tj and other bilinear terms in cluster amplitudes, such as Jso... 

As we can see, the QMMCC(2,6) method represents a considerable improvement over the LMMCC or CR-CCSD[T], CR-CCSD(T), and CR-CCSD(TQ),b methods. In addition to the linear terms in Y that are already present in the LMMCC or CR-CCSD[T] and CR-CCSD(T) models and that engage the triply excited moments (2) (see eqs (23)-<26)) and in addition to the quadratic (32 ) terms that are present in the CR-CCSD(TQ),b energy expression and that engage the tetraexcited moments (2) (14), the... 

CCSD(T) and CR-CCSD(TQ),b results for the potential energy curve of the N2 molecule, as described by the DZ basis set (113), shown in Table 1 and Figure 1. 

As expected, the QMMCC(2,4) method does not provide any significant improvements in comparison with the CR-CCSD(TQ),b approach. As shown in Table I, the QMMCC(2,4) and CR-CCSD(TQ),b results are essentially identical. In other words, we cannot ignore the terms in the QMMCC(2,6)... 

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. 

CR-CCSD(T) calculations of the 2 + 2-cycloaddition of cyclopentyne to ethylene indicate a highly exothermic (AGr298 = -68 kcalmol-1) reaction and a predominately... 

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