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Performance of Electronic Structure Methods for the Description of Hückel–Möbius Interconversions in Extended π‑Systems
journal contribution
posted on 2020-03-12, 20:43 authored by Tatiana Woller, Ambar Banerjee, Nitai Sylvetsky, Golokesh Santra, Xavier Deraet, Frank De Proft, Jan M. L. Martin, Mercedes AlonsoExpanded porphyrins
provide a versatile route to molecular switching
devices due to their ability to shift between several π-conjugation
topologies encoding distinct properties. DFT remains the workhorse
for modeling such extended macrocycles, when taking into account their
size and huge conformational flexibility. Nevertheless, the stability
of Hückel and Möbius conformers depends on a complex
interplay of different factors, such as hydrogen bonding, π···π
stacking, steric effects, ring strain, and electron delocalization.
As a consequence, the selection of an exchange–correlation
functional for describing the energy profile of topological switches
is very difficult. For these reasons, we have examined the performance
of a variety of wave function methods and density functionals for
describing the thermochemistry and kinetics of topology interconversions
across a wide range of macrocycles. Especially for hexa- and heptaphyrins,
the Möbius structures have a stronger degree of static correlation
than the Hückel and twisted-Hückel structures, and as
a result the relative energies of singly twisted structures are a
challenging test for electronic structure methods. Comparison of limited
orbital space full CI calculations with CCSD(T) calculations within
the same active spaces shows that post-CCSD(T) correlation contributions
to relative energies are very minor. At the same time, relative energies
are weakly sensitive to further basis set expansion, as proven by
the minor energy differences between the extrapolated MP2/CBS energies
estimated from cc-pV{T,Q}Z, diffuse-augmented heavy-aug-cc-pV{T,Q}Z
and explicitly correlated MP2-F12/cc-pVDZ-F12 calculations. Hence,
our CCSD(T) reference values are reasonably well-converged in both
1-particle and n-particle spaces. While conventional
MP2 and MP3 yield very poor results, SCS-MP2 and particularly SOS-MP2
and SCS-MP3 agree to better than 1 kcal mol–1 with
the CCSD(T) relative energies. Regarding DFT methods, the range-separated
double hybrids, such as ωB97M(2) and B2GP-PLYP, outperform other
functionals with RMSDs of 0.6 and 0.8 kcal mol–1, respectively. While the original DSD-PBEP86 double hybrid performs
fairly poorly for these extended π-systems, the errors drop
down to 1.9 kcal mol–1 for the revised revDOD-PBEP86-NL,
which eliminates the same-spin correlation energy. Minnesota meta-GGA
functionals with high fractions of exact exchange (M06-2X and M08-HX)
also perform reasonably well, outperforming more robust and significantly
less empirically parametrized functionals like SCAN0-D3.
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SOS-MP 2ω B 97Msame-spin correlation energy1- particleMinnesota meta-GGA functionalsrevDOD-PBEP 86-NLSCS-MP 3SCS-MP 2MP 3molparticle spaceserrors dropelectron delocalizationsteric effectsHuenergy profileπ- systemsCI calculationsMP 2kcal0-DCCSDπ- conjugation topologies encodingM 06-2Xenergy differencesMP 2-F calculationsstructure methodsM 08-HXDSD-PBEP 86topology interconversionsB 2GPSCANring strainRMSDDFT methodswave function methodstopological switchesparametrized functionalsElectronic Structure Methodsdensity functionals
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