Structural manifestation of the delocalization error of density functional approximations: C(4N+2) rings and C(20) bowl, cage, and ring isomers.
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The ground state structure of C(4N+2) rings is believed to exhibit a geometric transition from angle alternation (N < or = 2) to bond alternation (N > 2). All previous density functional theory (DFT) studies on these molecules have failed to reproduce this behavior by predicting either that the transition occurs at too large a ring size, or that the transition leads to a higher symmetry cumulene. Employing the recently proposed perspective of delocalization error within DFT we rationalize this failure of common density functional approximations (DFAs) and present calculations with the rCAM-B3LYP exchange-correlation functional that show an angle-to-bond-alternation transition between C(10) and C(14). The behavior exemplified here manifests itself more generally as the well known tendency of DFAs to bias toward delocalized electron distributions as favored by Huckel aromaticity, of which the C(4N+2) rings provide a quintessential example. Additional examples are the relative energies of the C(20) bowl, cage, and ring isomers; we show that the results from functionals with minimal delocalization error are in good agreement with CCSD(T) results, in contrast to other commonly used DFAs. An unbiased DFT treatment of electron delocalization is a key for reliable prediction of relative stability and hence the structures of complex molecules where many structure stabilization mechanisms exist.
Published Version (Please cite this version)10.1063/1.3445266
Publication InfoHeaton-Burgess, Tim; & Yang, Weitao (2010). Structural manifestation of the delocalization error of density functional approximations: C(4N+2) rings and C(20) bowl, cage, and ring isomers. J Chem Phys, 132(23). pp. 234113. 10.1063/1.3445266. Retrieved from https://hdl.handle.net/10161/3322.
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Philip Handler Distinguished Professor of Chemistry
Prof. Yang, the Philip Handler Professor of Chemistry, is developing methods for quantum mechanical calculations of large systems and carrying out quantum mechanical simulations of biological systems and nanostructures. His group has developed the linear scaling methods for electronic structure calculations and more recently the QM/MM methods for simulations of chemical reactions in enzymes.