Density functional studies of vibrational properties of HCN, H2O, CH2O, CH4 and C2H4

Small molecules provide a unique testing ground for post-Hartree-Fock methods because their harmonic force constants and dipole moment derivatives are known experimentally. We have compared several density functional theory (DFT) methods with various combinations of exchange and correlation functionals such as BLYP, B3LYP, BP86, B3P86, BPW91, B3PW91, BVWN, BVWN5, and BPL in combination with several basis sets for HCH. Most density functional results provide impressive agreement with the experiment for geometries and vibrational frequencies. Methods with B3 exchange functional consistently overestimate diagonal force constants, while other methods underestimate the CH stretching force constant in HCN. However, all calculated dipole moment derivatives of HCN, including HF, DFT, and QCISD, show less accurate agreement with experiment than for vibrational frequencies. The results show a strong dependency on the choice of the basis set and on the form of the density functional, On the basis of the extensive basis set and DFT potential dependency studies on HCN, we have applied only the three most promising DFT's to a calculation of other small molecules in order to generalize what we have observed. Calculations on H2O CH2O, CH4, and C2H4 molecules provide relatively accurate predictions of dipole moment derivatives. DFT calculations allow unequivocal sign assignments in the determination of atomic polar tensors (APT) of these molecules including a new assignment of signs of the dipole moment derivatives for C2H4 and confirming earlier assignments of the H2CO and CP4 of Person et al. These results further support DFT as a means for obtaining reasonable dipole moment derivatives.