where, is the total energy of monolayer structures, and E and E 1 / 2 are energies of Cr and S/Se atoms in their corresponding stable bulk phases with N number of atoms. In cohesive energy expression, E represents the total energy of a monolayer and represents the total energy of isolated atoms with as the number of a specific atom per monolayer. Further, we adopt the frozen phonon method implemented in the Vibra package of SIESTA for the calculation of phonon dispersion. We obtain an accurate force matrix with 5 × 5 × 1 supercell for each monolayer.