First, recent remarkable progress in molecular orbital theory was reviewed in connection with the energy gradient method, with which the structure of the activated complex and the reaction coordinate can be determined. It should be noted that these data can not be obtained from any experiments.Next, the above mentioned method was applied to the sodium ion channel to clarify the mechanism of ion selectivity of the channel. It is found that an ion with lower activation free energy has larger pemearbility. Then, the electronic structures of DNA base pairs, A-T and A*-T* were discussed together with the reliability of molecular orbital theory. Finally, the elongation method was briefly illustrated.