A BENCHMARK VIBRATIONAL POTENTIAL SURFACE - GROUND-STATE BENZENE

The unique symmetry constraints present in benzene, allowing extensive experimental and theoretical and analyses, establish ground-state benzene as a benchmark for theoretical and empirical potential surface methods. The empirical harmonic vibrational potential surface generated from frequencies described here accurately simulates the vibrational properties of benzene and its isotopomers (in general within 0.1% in harmonic frequencies, 0.01 units for accurately measured Coriolis constants, 10% in infrared, Raman, and two-photon intensities). Present large-scale ab initio calculations for benzene do not reproduce the empirical force constants with the required 0.01 mdyn/angstrom for spectroscopic utility. Anharmonic frequency corrections are important in generating the force field. Mode frequencies do not provide a particularly good test of force constant accuracy, but eigenvector-based quantities, such as intensities or vibration-rotation constants, incisively test and distinguish between multiple force constant solutions.