Comparison between Optimized Geometries and Vibrational Frequencies Calculated by the DFT Methods

Optimized geometries, vibrational frequencies, and scale factors were calculated for furan and thiophene with the HF, MP2, LDA, BVWN, BLYP, and B3LYP methods of theory using the 6-31G**, cc-pVDZ, and cc-pVTZ basis sets. The agreement between the optimized and experimental geometries was in the order B3LYP, MP2, LDA, BVWN, BLYP, then HF. The calculated frequencies by the unscaled BVWN force field had the smallest average error in the mid-IR region, but using one-scale-factor scaling, those calculated by the scaled B3LYP force field had the lowest average error. Using one-scale-factor scaling, scale factors of 0.82, 0.89, 0.98, 0.93, 0.96, and 0.96 were obtained by the HF, MP2, BLYP, B3LYP, LDA, and BVWN force fields, respectively, using the 6-31G** basis set. The effect of the basis set on the calculated bond angles, frequencies, and scale factors by the DFT methods was minor, but except with the LDA method, the agreement between the calculated and experimental bond lengths can be arranged in the order cc-pVDZ, 6-31G**, then cc-pVTZ basis set.