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 m odes, omega(4) and omega(5) (and to a lesser extent omega(17)), exhibi t a pathological basis set dependence due to basis set superposition e rror. Using an atomic natural orbital (ANO) basis set of [4s3p2d1f/4s2 p] quality, the best available experimentally derived harmonic frequen cies can be reproduced with an RMS error of 6 cm(-1) without any empir ical corrections. We strongly recommend the use of ANO basis sets for accurate frequency calculations on unsaturated and aromatic systems. O ur best estimate for the equilibrium geometry is r(e)(CC)=1.392(2), r( e)(CH)=1.081 Angstrom. (C) 1997 Published by Elsevier Science B.V.