Structural and electronic properties of polyacetylene and polyyne from hybrid and Coulomb-attenuated density functionals

Michael J. G. Peach, Erik I. Tellgren, Paweł Sałek, Trygve Helgaker, David J. Tozer, J. Phys. Chem. A, 111, 11930–11935, 2007.


In the first of our collaborations with Prof Trygve Helgaker, while he was on Sabbatical leave in Durham from the University of Oslo, we applied CAM-B3LYP to investigate structural and electronic properties in oligomers.

This paper derived from observations by Jacquemin and co-workers that suggested the BHHLYP functional was often preferable to B3LYP for the evaluation of bond-length alternation (BLA) parameters in oligomeric species. We showed that the CAM-B3LYP functional was able to simultaneously achieve excellent BLA parameters, and excitation energies, across a series of polyacetylene and polyyne oligomers. We also developed an approach for extrapolating excitation energies based on HO–LU orbital energy gaps.

Further information, including details of subsequent work in this area, can be found on  research page. For the abstract, and access to the full text, see below.


The bond length alternation (BLA), the highest-occupied−lowest-unoccupied (HO−LU) orbital energy gap, and the corresponding excitation energy are determined for trans-polyacetylene (PA) and polyyne (PY) using density functional theory. Results from the Coulomb-attenuated CAM-B3LYP functional are compared with those from the conventional BHHLYP and B3LYP hybrid functionals. BLA values and HO−LU gaps are determined using both finite oligomer and infinite chain calculations, subject to periodic boundary conditions. TDDFT excitation energies are determined for the oligomers. The oligomer excitation energies and HO−LU gaps are then used, in conjunction with the infinite chain HO−LU gap, to estimate the infinite chain excitation energy. Overall, BHHLYP and CAM-B3LYP give BLA values and excitation energies that are larger and more accurate than those obtained using B3LYP. The results highlight the degree to which excitation energies can be approximated using the HO−LU gapsat the infinite limit, this approximation works well for B3LYP, but not for the other functionals, where the HO−LU gap is significantly larger. The study provides further evidence for the high-quality theoretical predictions that can be obtained from the CAM-B3LYP functional.

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