The harmonic frequencies of benzene. A case for atomic natural orbital basis sets

The geometry and harmonic force field of benzene have been computed at the CCSD(T) level with basis sets of spdf quality. Two out-of-plane modes, ω4 and ω5 (and to a lesser extent ω17), exhibit a pathological basis set dep to basis set superposition error. Using an atomic natural orbital (ANO) basis set of [4s3p2d1η/4s2p] quality, the best available experimentally derived harmonic frequencies can be reproduced with an RMS error of 6 cm−1 without any empirical corrections. We strongly recommend the use of ANO basis sets for accurate frequency calculations on unsaturated and aromatic systems. Our best estimate for the equilibrium geometry is re(CC) = 1.392(2), re(CH) = 1.081 Å.