Balancing dynamic and nondynamic correlation for diradical and aromatic transition states. A renormalized coupled-cluster study of the cope rearrangement of 1,5-hexadiene

The denominator D resulting from the CR-CCSD(T) calculations correlates with the degree of diradical character of the system of interest. The CCSD(T) approach often regarded as the gold standard of electronic structure theory favored pathways through diradical structures.