THE HARMONIC FORCE-FIELD OF BENZENE - A LOCAL DENSITY FUNCTIONAL-STUDY

The harmonic force field of benzene has been calculated by a method based on local density functional theory (LDF). The calculations were carried out employing a triple zeta basis set with triple polarization on hydrogen and double polarization on carbon. The LDF force field was compared to the empirical field due to Ozkabak, Goodman, and Thakur [A. G. Ozkabak, L. Goodman, and S. N. Thakur, J. Phys. Chem. 95, 9044 (1991)], which has served as a benchmark for theoretical calculations as well as the theoretical field based on scaled Hartree-Fock ab initio calculation due to Pulay, Fogarasi, and Boggs [P. Pulay, G. Fogarasi, and J. E. Boggs, J. Chem. Phys. 74, 3999 (1981)]. The calculated LDF force field is in excellent qualitative and very good quantitative agreement with the theoretical field proposed by Pulay, Fogarasi, and Boggs as well as the empirical field due to Ozkabak, Goodman, and Thakur. The LDF field is closest to the values of Pulay and co-workers in those cases where the force constants due to Pulay, Fogarasi, and Boggs and to Ozkabak, Goodman, and Thakur differ in sign or magnitude. The accuracy of the LDF force field was investigated by evaluating a number of eigenvalue and eigenfunction dependent quantities from the the LDF force constants. The quantities under investigation include vibrational frequencies of seven isotopomers, isotopic shifts, as well as absorption intensities. The calculations were performed at both theoretical optimized and approximate equilibrium reference geometries. The predicted frequencies are usually within 1%-2% compared to the empirical harmonic frequencies. The least accurate frequency deviates by 5% from the experimental value. The average deviations from the empirical harmonic frequencies of C6H6 and C6D6 are 16.7 cm-1 (1.5%) and 15.2 cm-1 (1.7%), respectively, not including CH stretching frequencies, in the case where a theoretical reference geometry was used. The accuracy of the out-of-plane force field is especially remarkable; the average deviations for the C6H6 and C6D6 frequencies, based on the LDF force field, are 9.4 cm-1 (1.2%) and 7.3 cm-1 (1.2%), respectively. The absorption intensities were not predicted as accurately as it was expected based on the size of the basis set applied. An analysis is provided to ensure that the force constants are not significantly affected by numerical errors due to the numerical integration scheme employed.