The harmonic vibrational frequencies and the geometry of the 12C6H6

The harmonic frequencies ωest of 12C6H6 calculated by combining the anharmonic constants from a DFT quartic force field with the experimental fundamentals, are compared with harmonic frequencies, ωcalc, obtained with different theoretical methods. One set of ωcalc was derived by Martin, Taylor and Lee by ab initio CCSD(T) theory, the others have been calculated by us by means of the density funcional theory DFT with B3LYP, B3PW91, B3P86 and mPW1PW hybrid functionals. The mean absolute deviation between ab initio ωcalc and ωest is 6.8 cm−1 while between DFT ωcalc and ωest is between 8.6 and 10.5 cm−1. Moreover the DFT ωcalc show a small systematic underestimation of the CH stretching vibrations.