Optical bandgaps from ab initio methods: an examination of the singlet-triplet approximation

Abstract

The estimation of polymer bandgaps by extrapolation of singlet-triplet transition energies has been examined for polyacetylene. Singlet-triplet and singlet-singlet transition energies were calculated with a variety of basis sets for selected oligomers. The singlet-triplet energy shows only small ( < 0.1 eV) variations with basis set, whereas the singlet-singlet energy, for smaller molecules, requires polarization and diffuse functions to be adequately described. The importance of polarization and diffuse functions diminishes for longer oligomers, suggesting that, for the infinite systems approximating real polymers, the diffuse functions are not important, especially for the triplet state. Even with C 10H 12, the polarization functions still contribute ≈0.05 eV.