Optimized geometries and total energies for the 2,3,7,8-tetrachlorinated dibenzo-p-dioxin (TCDD) molecules were calculated using the Gaussian 92 system of programs at two ab initio MO levels : RHF/6-31G and RHF/6-31G*. In addition to the above levels, the corresponding basis sets, including diffuse function, were also used. The fully-optimized geometry is given for the TCDD molecule. The results were compared and agree closely with the observed structures, which wire obtained by X-ray spectrometry. Harmonic vibrational frequencies are calculated at the above-mentioned levels of theory on the basis of the optimized geometries. Comparison with experimental IR results is discussed. It is predicted from vibrational analyses that the butterfly flapping motion of two benzo-planes is a motion having a very low fundamental frequency (3.6 cm(-1)). For those modes which are IR inactive, the predicted Raman frequencies are reported. Some ambiguity concerning the dynamic behavior has also been investigated in terms of the potential energy curve (PEC) as a function of the folding angle (theta), to specify the butterfly flapping motion of the two benzo-planes. It is shown that the potential has little curvature near the minimum. The small curvature of the PEC appears to be consistent with the far-infrared 3.6 cm(-1) frequency of the flapping motion.