Abstract: Using the B3P86 method of density functional theory, the author optimized the ground state stable geometric configuration of S-Serine (S-Ser) molecules and charged ions at the level of 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 S-Ser molecule and ion. The results show that the structural parameters of the SSer molecule and S-Ser-2- change with the increase of the number of electrons captured in the molecular system, such as the bond length R is the most obvious change. The molecular orbital (MO) combined with natural transition orbital (NTO) characteristic analysis method can better describe the electronic excitation characteristics of each excited state of S-Ser system, and the identification effect for Rydberg excitation characteristics is better.

Key words: serine (Ser) molecule, ion, low excited state, orbital transition