Abstract
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.
Highlights
The popular DLPNOCCSD(T) approach, in which off-diagonal Fock matrix elements are neglected in the (T) contribution, corresponds to an approximation to canonical CCSD(T0)
We find that DLPNO-CCSD(T1) for these systems requires about twice the CPU time for DLPNO-CCSD(T0): the fairly high I/ O bandwidth required for (T1) required running on nodes with high-speed local SSD scratch disk volumes
For the entire database of expanded porphyrins, we find an RMSD of 1.2−1.3 kcal mol−1 both for pair natural orbitals (PNOs)-LCCSD(T) on Normal settings and for DLPNO-CCSD(T1) on Tight settings
Summary
Expanded porphyrins have drawn much attention over the past few decades due to their facile redox interconversions, novel metal coordination behaviors, versatile electronic states, and conformational flexibility. The latter is responsible for the rich chemistry associated with such systems, which has led to various applications such as near-infrared dyes, nonlinear optical materials, magnetic resonance imaging contrast agents, and molecular switches. Contrary to the parent porphyrin, expanded porphyrins are flexible enough to undergo conformational changes, which correspond to distinct π-conjugation topologies (Hückel, Möbius, and twistedHückel/figure-eight) encoding different chemical and physical properties (Scheme 1).6,7Such changes may involve a Hückel−Möbius aromaticity switch within a single molecule, which can be induced by, inter alia, an appropriate solvent, pH, temperature, and metalation conditions. these Hückel−Möbius aromaticity switches have already been recognized for their potential applications in molecular optoelectronic devices. Additional applications for expanded porphyrins, including conductance switching devices and efficient nonlinear optical switches, have recently been covered in the literature. Expanded porphyrins have drawn much attention over the past few decades due to their facile redox interconversions, novel metal coordination behaviors, versatile electronic states, and conformational flexibility.1 The latter is responsible for the rich chemistry associated with such systems, which has led to various applications such as near-infrared dyes, nonlinear optical materials, magnetic resonance imaging contrast agents, and molecular switches.. Contrary to the parent porphyrin, expanded porphyrins are flexible enough to undergo conformational changes, which correspond to distinct π-conjugation topologies (Hückel, Möbius, and twistedHückel/figure-eight) encoding different chemical and physical properties (Scheme 1).6,7 Such changes may involve a Hückel−Möbius aromaticity switch within a single molecule, which can be induced by, inter alia, an appropriate solvent, pH, temperature, and metalation conditions..
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