Abstract: Using the method of density functional B3LYP, we optimized the ground state stable geometries of  the gas phase alanine (Ala) molecules
 and charged ions at the level of 6-311++G(-d,p-) basis set. The method of time dependent density functional theory (TD-DFT) was used to study the low excited state properties of the gas phase Ala molecules and ions. The results show that the bond length gradually decreases, the excitation energy of the first excited state  decreases, the fluorescence wavelength increase sharply, and the number of electronic transition orbit of S6 increases with the increases of the number of electrons obtained by molecules.

Key words: transition, alanine molecule； ion； low excited state； orbital