**2. Computational method**

We perform calculations on BeSe by using the CASTEP code [10]. The energy Ecut of 660 eV and k-points of 8 8 8 using Monkhorst-Pack scheme [11] ensure well

convergence. We use 20 20 20 k-points in the computation of optical parameters. The GGA, LDA, and HSE hybrid functional [12, 13] treat the exchange-correlation potential. Broyden-Fletcher-Goldfarb-Shanno (BFGS) minimization technique [14] determines the structural parameters. The tolerance of geometry optimization was a total energy 5 <sup>10</sup><sup>6</sup> eV/atom, maximum ionic Hellmann-Feynman force 0.01 eV/Å, and maximum stress 0.02 eV/Å<sup>3</sup> .
