Improved Geometries and Frequencies with the PFD-3B DFT Method.

Bond lengths have been calculated for a test set of 120 diatomic species, including all homonuclear diatomics, hydrides, fluorides, and oxides for elements H through Kr for which experimental data is available for comparison. The performance of the PFD-3B functional is significantly better than competitive DFT methods. The rms error in bond lengths is reduced to 0.01 Å using a moderate size 3Za1Pa + f triple-ζ basis set, with the rms error in harmonic vibrational constants, ω e , equal to 38 cm -1 . A very small 2ZP0H basis set is sufficient to calculate anharmonic constants, ω e X e , within ±4 cm -1 . The rotational constants, B e , agree with experiment to within ±2%, and the vibration-rotation coupling constants, α e , agree within 10%. The calculated vibrational zero-point energy, ZPE, agrees with experiment to within ±0.06 kcal mol -1 for the diatomic test set, and the error increases to just ±0.11 kcal mol -1 for a set of 12 small polyatomic species. Comparison of a detailed anharmonic analysis of the twisted ethylene cation to the PFI-ZEKE experimental data illustrates the reliability of the PFD-3B for atypical structures.