2.2. Calculational details
All of the calculations are carried out using plane-wave pseudopotential DFT+U method in “CASTEP” module, which is developed by Accelrys Company[21], the exchange correlation energy were treated based on the GGA with PBE functional[22], with the considerations of spin-orbit polarization. In the simulation, the core electrons were treated with the ultrasoft pseudopotential, and the configurations of valence electrons were selected as Bi 6s26p3, O 2s22p4, Br 4s24p5, Sc 3d14s2, Ti 3d24s2, V 3d34s2, Cr 3d54s1, Mn 3d54s2, Fe 3d64s2, Co 3d74s2, Ni 3d84s2, Cu 3d104s1, Zn 3d104s2. The plane-wave cut-off energy of 500 eV and a Monkhorst-Pack 3×3×3 \(k\)-point grid were used. In addition, the convergence standards were set as the follows: the force and stress were less than 0.5 eV/Å and 0.1 Gpa, respectively as well as the energy change was less than 2 × 10−5eV/atom[18]. More importantly, we adopted DFT+U approach to overcome the disadvantage of GGA-PBE due to the nonnegligible d-d transition of transition metal[23, 24]. As we all know, a finite onsite Coulomb interaction is essential for accurately describing atoms containing d electrons[25], so the on-site Coulomb interaction with U=4.0 eV is used in the calculations[26]. In order to make the calculated optical properties results consistent with the actual situation, “scissors operators” technology[27]was adopted, and the correction factor was set as 0.8 eV in our work, which is based on the deviation of the calculated values and the experimental data of average band gap.