Abstract: The geometry of R-threonine (R-Thr) molecules in the implicit solvent methanol was optimized at the B3LYP/6-31G(d) basis set level, based on this structue, the time-dependent density functional theory (TDDFT) method was used to calculate the electronic excitation at the Mo6-2X/6-311G(d),TZVP,def2-TZVP basis set level, and the valence layer electronic excitation type of R-Thr-CH₃OH molecular system was studied  by the method of identifying the type of natural transition orbit (NTO). The results show that the judgement results of TZVP and def2-TZVP basis sets are basically consistent, 6-311G(d) basis set is compared with the other two basis sets, there is a big difference in the judgment results of excitation type. The combination of NTO type analysis and  Δr indes analysis of theoretical data has practical significance for the identification of the electronic excitation type and excitation mode of R-Thr-CH₃OH molecular system.

Key words: threonine, electronic excitation type, natural transition orbit (NTO)； , implicit solvent methanol