Eight medium-sized carbon-containing molecules: linear carbon chains C-n (n = 6-9), triacetylene (C6H), tetracyanoethylene (C6N4), 1,1,1-trifluoroacetone enolate (CF3CHCHO), and C4O have been studied using six different density functional or hybrid Hartree-Fock density functional methods with a double-zeta basis set with polarization and diffuse functions (DZP++). Optimized geometries, harmonic vibrational frequencies, and adiabatic electron affinities were estimated and compared to known experimental values. The harmonic vibrational frequencies showed an overall agreement with experimental fundamentals of approximately 4-6% with one exception, the BHLYP functional. Average absolute errors in electron affinities estimated with the BP86, BLYP, and B3LYP functionals all show agreement of better than 0.2 eV with experiment and provide a viable method of predicting electron affinities for molecules of the same type as studied here.