COMPARISON AND SCALING OF HARTREE-FOCK AND DENSITY-FUNCTIONAL HARMONIC FORCE-FIELDS .1. FORMAMIDE MONOMER

We have compared force constants of formamide calculated by a wide range of ab initio quantum chemical methods and basis sets. Force fields calculated by local and gradient-corrected density functional theory (DFT) methods were found to be closer to MP2/6-31G** values than Hartree-Fock (HF) values. A comparison of calculated frequencies and infrared intensities with experimental values indicated better performance of the DFT methods over HF and even MP2. Several scaling schemes for interaction force constants were evaluated, as well as the effect of the variation of interaction force constants with theoretical method on the calculated frequencies, IR intensities, and normal modes.