Performance of Localized Coupled Cluster Methods in a Moderately Strong Correlation Regime: Hückel–Möbius Interconversions in Expanded Porphyrins

Localized
orbital coupled cluster theory has recently emerged as
a nonempirical alternative to DFT for large systems. Intuitively,
one might expect such methods to perform less well for highly delocalized
systems. In the present work, we apply both canonical CCSD­(T) approximations
and a variety of localized approximations to a set of flexible expanded
porphyrinsmacrocycles that can switch between Hückel,
figure-eight, and Möbius topologies under external stimuli.
Both minima and isomerization transition states are considered. We
find that Möbius­(-like) structures have much stronger static
correlation character than the remaining structures, and that this
causes significant errors in DLPNO-CCSD­(T) and even DLPNO-CCSD­(T1) approaches, unless TightPNO cutoffs are employed. If sub-kcal
mol–1 accuracy with respect to canonical relative
energies is required even for Möbius-type systems (or other
systems plagued by strong static correlation), then Nagy and Kallay’s
LNO-CCSD­(T) method with “tight” settings is the suitable
localized approach. We propose the present POLYPYR21 data set as a
benchmark for localized orbital methods or, more broadly, for the
ability of lower-level methods to handle energetics with strongly
varying degrees of static correlation.