As shown in Fig. , the similarity of ethylene and ethane, propane and propene, and cyclohexane and benzene in the A values strikes the common knowledge of molecular structure. Unlike the rotatable σ bond, a C-C π bond is usually recognized as a rigid and symmetric bond; thereby, as an example, benzene should have exhibited excellent symmetry relative to cyclohexane. However, as shown in Fig. , the difference in the A values is not as significant as anticipated. A cyclohexane molecule is commonly recognized as a soft regular-hexagon frame that the conformations of chair type and boat type can form due to the rotation of C-C σ bonds. In contrast, a benzene molecule is a rigid regularhexagonal pie fixed by a grand π bond. Therefore, the operation number of a benzene ring should be figured out readily, i.e., n = 12 (1C2+3C3+1C6+6C2+E). Correspondingly, above B.P., A=4.3072 corresponds to n=9, while below B.P., A=5.2 corresponds to n=11. However, for cyclohexane, the max. n = 7 (3 boat types×1 C2 + 3 chair types×1C2 +E). It needs additional symmetry operations. Based on effective packing and entropy, the mirror operation can be added to the operations of cyclohexane under the condition that it does not overlap the rotation operation. By this means, n=3 boat types × (1 C2 + 1 σ 2 ) + 3 chair types × (1C2 +1 σ 2 ) +E =13. Unfortunately, such a treatment is not suitable for ethylene. Therefore, as mentioned above, for N2, etc., π bonds must be rotatable to satisfy n/2≥A. There are only 3 C2 operations for CH2=CH2, but the max is A=5.1978, corresponding to n≥11. In contrast, ethane has the operations of 3C3+9C2+3C2+E=16, thus the max. A=6.2299 is proper.