Abstract: The charge transfer between molecular fragments was calculated based on components of molecular orbit (MO) and natural transition orbit (NTO), we first optimized the geometric configuration of threonine (Thr) molecule at 6-31G(d) basis set under implicit solvent H₂O by using the CAMB3LYP method of density functional theory. Then, we calculated the electron excitation of time dependent density functional theory (TDDFT) under the same theoretical method, and gave the results of comparing the charge transfer characteristics between fragments during the electron excitation of Thr molecular system under implicit solvent H₂O. The results show that in the S1—S5 excited state, only a pair of MO32→MO33 transition orbits are dominant in the S2, and the charge transfer can be discussed by analyzing the orbital components of the orbit, 
charge transfer of other excited states can be discussed by NTO analysis method. The main contribution of charge transfer from S0 to excited states S1,S3 and S4 is NTO32→NTO33 orbit, which is consistent with the qualitative results of 
Hirshfeld and Becke methods, and the quantitative results are slightly different.

Key words: threonine (Thr), molecular orbit (MO), natural transition orbit (NTO), charge transfer