Context <p>This study aims to clarify the linear response excitation mechanisms and characteristics of the explosive 1-methyl-3,5-dinitro-1,2,4-triazole (MDNT) in the implicit solvent dimethyl sulfoxide (DMSO) when transitioning from the ground state to the 20 lowest-energy excited states. In the ground state MDNT molecule, there exists a steric hindrance effect within the triazole ring and between the nitro oxygen (O) and the azole ring nitrogen (N). Ahydrogen bond dominated by dispersion interaction is formed between the nitro O and the methyl hydrogen (H). Among the 20 low-energy excited states, S5, S7, S9, S10, S11, and S12 are significant bright states (with an oscillator strength of 0.01), and the remaining 14 are dark states (weakly allowed transitions, not observable in conventional UV-vis spectra). Note that this differs from the d–d transitions in transition metal complexes, where an <i>f</i> of 0.001 may still be weakly observable. The lifetimes of the six bright states are all in the nanosecond ranges (S10 is the longest, reaching 4.93 ns; S11 is the shortest, only 0.7 ns). S11 requires an excitation energy of 6.47 eV and corresponds to the maximum absorption peak. In terms of the excitation mechanism, S0 → S5/S7/S12 is a C–N π → N–O π charge transfer excitation (CT), S0 → S11 is a C–N π → N–O π local excitation (LE), and S0 → S9/S10 is a local excitation (LE) of the lonepair electrons n → N–O π* of the nitro O. This study provides a theoretical reference for the research on the electronic excitation mechanism and decomposition behavior of energetic materials.</p> Method <p>The Gaussian16 software is used for optimization and calculation, and the Multiwfn and VMD programs are applied for further processing and visualization. Using the time-dependent density functional theory (TDDFT) at the M06-2X/6-311G(d) level, the linear response electron excitation of MDNT in implicit solvent DMSO was investigated.</p>