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Localized orbital coupled cluster theory has recently emerged as annonempirical alternative to DFT for large systems. Intuitively, one mightexpect such methods to perform less well for highly delocalized systems. In thepresent work, we apply both canonical CCSD(T) and a variety of localizedapproximations thereto to a set of expanded porphyrins -- systems that canswitch between H\"uckel, figure-eight, and M\"obius topologies under externalstimuli. Both minima and isomerization transition states are considered. Wefind that M\"obius(-like) structures have much stronger static correlationcharacter than the remaining structures, and that this causes significanterrors in DLPNO-CCSD(T) and even DLPNO-CCSD(T1) approaches, unless TightPNOcutoffs are employed. If sub-kcal/mol reproduction of canonical relativeenergies is required even for M\"obius-type systems (or other systems plaguedby strong static correlation), then Nagy and Kallay's LNO-CCSD(T) method with"tight" settings can provide that, at much greater computational expense thaneither the PNO-LCCSD(T) or DLPNO-LCCSD(T) approaches but with still a muchgentler CPU time scaling than canonical approaches. We would propose thepresent POLYPYR21 dataset as a benchmark for localized orbital methods, or morebroadly, for the ability of lower-level methods to handle energetics withstrongly varying degrees of static correlation.

Performance of Localized Coupled Cluster Methods in a Moderately Strong Correlation Regime: H\"uckel-M\"obius Interconversions in Expanded Porphyrins