The energies of the temporary anion states of 1,4-benzenedithiol (BDT) are determined by electron scattering and compared with those computed by density functional theory (DFT) and Hartree-Fock (HF) methods. Using semi-empirical scalings derived from the ionization energies and electron affinities of a series of phenyl-ethynyl compounds, we compute the HOMO-LUMO energy gap of BDT. We show that without such scalings the gap is substantially underestimated using DFT and overestimated using HF methods. Scaled gaps are also determined for several related compounds and compared with experimental measurements in the literature. (C) 2008 Elsevier B. V. All rights reserved.