DESIGN OF LOW-BAND-GAP POLYMERS EMPLOYING DENSITY-FUNCTIONAL THEORY -HYBRID FUNCTIONALS AMELIORATE BAND-GAP PROBLEM

Band gaps in solids and excitation energies in finite systems are unde restimated significantly if estimated from differences between eigenva lues obtained within the local spin density approximation (LSDA). In t his article we present results on 20 small-and medium-sized pi-systems which show that HOMO-LUMO energy differences obtained with the B3LYP, B3P86, and B3PW91 functionals are in good agreement with vertical exc itation energies from UV-absorption spectra. The improvement is a resu lt of the use of the exact Hartree-Fock exchange with hybrid methods. Negative HOMO energies and negative LUMO energies do not provide good estimates for IPs and EAs. Ln contrast to Hartree-Fock theory, where I Ps are approximated well and EAs are given poorly, DFT hybrid methods underestimate IPs and EAs by about the same amount. LSDA yields reason able EAs but poor IPs. (C) 1997 John Wiley & Sons, Inc.