Certain molecules, which are intimately related, have been classified into three types according to the general characteristics with respect to the internal rotation. By symmetry analysis of these three types of molecules the general forms of potential functions have been obtained. Applying the results obtained, and the additive property of interaction energies between chemical bonds, we have calculated the potential functions of 85 molecules. All the calculated results agree well with the experimental data. Besides, the number of isomers of internal rotation and the stable configurations in relation to both the meso and the active forms of two molecules CC1FH-CC1FH and C(CH3)C1H-C(CH3)C1H have been predicted .