1：Experimental Design and Method Selection:

The study uses the APW + lo method within the DFT framework as implemented in the WIEN2K simulation package. The exchange-correlation interaction is described by the GGA-PBE functional. The Brillouin zone is sampled with 1000 k-points using the tetrahedron method. Convergence criteria include a total energy difference of less than 0.0001 Ry between iterations, with wave function expansion parameters lmax = 10 and RMTmin * kmax = 7, and Gmax = 12 (a.u.)^{-1} for charge density Fourier expansion.

2：0001 Ry between iterations, with wave function expansion parameters lmax = 10 and RMTmin * kmax = 7, and Gmax = 12 (a.u.)^{-1} for charge density Fourier expansion. Sample Selection and Data Sources:

2. Sample Selection and Data Sources: The monolayer SnSe structure is modeled with a unit cell and vacuum region height over 20 ? to avoid spurious interactions. Lattice constants are set based on previous studies (a = 4.298 ?, b = 4.372 ?).

3：298 ?, b = 372 ?). List of Experimental Equipment and Materials:

3. List of Experimental Equipment and Materials: Computational software WIEN2K is used for simulations. No physical equipment is mentioned.

4：Experimental Procedures and Operational Workflow:

Strains (zigzag εzz, armchair εac, biaxial εb) are applied by varying lattice constants from -12% to 12%. Electronic band structures, density of states, and optical properties (dielectric function, absorption coefficient, etc.) are calculated under different strain conditions.

5：2%. Electronic band structures, density of states, and optical properties (dielectric function, absorption coefficient, etc.) are calculated under different strain conditions. Data Analysis Methods:

5. Data Analysis Methods: Data analysis involves plotting and comparing band structures, DOS, and optical spectra. Kramers-Kronig relations are used to derive optical properties from dielectric tensors.