EFFECT OF THE REFERENCE GEOMETRY AND THE EXCHANGE-CORRELATION FUNCTIONAL ON THE VIBRATIONAL FREQUENCIES CALCULATED BY DENSITY-FUNCTIONAL METHODS - THE EXAMPLES OF BENZENE AND NICKEL, CHROMIUM, AND IRON CARBONYLS

The vibrational frequencies of benzene, Ni(CO)(4), Cr(CO)(6), and Fe(CO)(5) have been calculated with the local density approximation (LDA) at various reference geometries. The reference geometries were taken from experiment and from geometry optimizations based on the LDA method as well as LDA augmented with nonlocal exchange-correlation corrections (LDA/NL). We found that the LDA method reproduces the observed harmonic frequencies remarkably well at the experimental reference geometry and most of the error at the LDA optimized structure can be related to reference geometry effects. For benzene, Cr(CO)(6), and Ni(CO)(4), the average deviations (and average percentage deviations) were 15.3 cm(-1) (1.32%), 5.1 cm(-1) (1.55%), and 7.0 cm(-1) (2.34%) at the experimental reference geometry, respectively. At the LDA optimized geometry, the average deviations were 32.6 cm(-1) (1.80%), 29.4 cm(-1) (7.56%), and 33.9 cm(-1) (10.6%), respectively. For Fe(CO)(5), the best agreement was found at the LDA/NL optimized reference geometry rather than the experimental structure, and this was attributed to significant uncertainties in the experimental structure. A review is provided of the frequency assignments for the iron pentacarbonyl spectra based on the calculated results, and some alternative assignments are suggested. We found no significant difference between the vibrational frequencies of the metal carbonyls calculated by local and nonlocal DFT methods, provided the reference geometries were the same.