Abstract: Using the method of density functional B3LYP, we optimized ground state stable geometries of the gas phase cysteine (Cys) molecules and charged ions at the level of the 6-311++G(-d,p-) basis set. At the same time, the method of time dependent density functional theory (TD-DFT) was used to study the
low excited state properties of the gas phase Cys molecules and ions. The results show that the energy of the system increases gradually, the difference between S1 excited state energy and ground state energy decreases, molecular fluorescence wavelength increases from 239.35 nm to 1 895.82 nm, and the number of electronic transition orbit of S7 excited state decreases with the increases of the number of electrons obtained by molecules.

Key words: cysteine (Cys) molecule, orbital transition, low excited state, ion