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 ba sis sets. The agreement between the optimized and experimental geometr ies was in the order B3LYP, MP2, LDA, BVWN, BLYP, then HF. The calcula ted frequencies by the unscaled BVWN force field had the smallest aver age error in the mid-IR region, but using one-scale-factor scaling, th ose 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 KF, MP2, BLYP, B3LYP, LD A, and BVWN force fields, respectively, using the 6-31G* basis set. T he effect of the basis set on the calculated bond angles, frequencies, and scale factors by the DFT methods was minor, but except with the L DA method, the agreement between the calculated and experimental bond lengths can be arranged in the order cc-pVDZ, 6-31G*, then cc-pVTZ ba sis set.