Ground-state potential energy surfaces involving bond

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. 

The ab initio spin-coupled valence bond (SCVB) approach continues to provide accurate ground and excited state potential energy surfaces for use in a variety of subsequent applications, with particular emphasis on intermolecular forces and reactive systems. The compactness of the various wavefunctions allows direct and clear interpretation of the correlated electronic structure of molecular systems. Recent developments, in the form of SCVB and MR-SCVB, involve the optimization of virtual orbitals via an approximate energy expression. These improved virtuals lead to still higher accuracy for the final variational wavefunctions, but with even more compact wavefunctions. 

---