“…In situations where degeneracy is commonly encountered (e.g., when crossing transition states, when accessing excited states, or when breaking or forming bonds), CCSD(T) can fail [28], and in these cases one can instead turn to new generations of CC methods, such as the left-eigenstate completely renormalized (CR) CC method [29][30][31][32] called CR-CC(2,3). The CR-CC(2,3) approach provides an accurate description of single-bond-breaking processes [33,34], biradical and magnetic molecules [35,36], and transition states [37,38]. Excited states can be accessed in a straightforward manner using the equation of motion (EOM) CC formalism [39][40][41][42], resulting in the CR-EOMCC(2,3) approximation and its size-intensive variant δ-CR-EOMCC (2,3) [31,[43][44][45][46].…”