  We report first-principles calculations, using the full potential linear
augmented plane wave method, on six lead halide semiconductors, namely,
CH3NH3PbI3, CH3NH3PbBr3, CsPbX3 (X=Cl, Br, I), and RbPbI3. Exchange is modeled
using the modified Becke-Johnson potential. With an appropriate choice of the
parameter that defines this potential, an excellent agreement is obtained
between calculated and experimental band gaps of the six compounds. We comment
on the possibility that the cubic phase of CsPbI3, under hydrostatic pressure,
could be a topological insulator.
