2D monolayer organic-inorganic hybrid halide perovskites A 2 MX 4 (Figure and Figure ) increase with the atomic number of halide elements, due to the differences in the ionic radii and the electronegativity of halide elements (F > Cl > Br > I). Since a fortuitous cancellation of errors induced by the counterbalance between GGA's natural underestimation of bandgap and the lack of spin-orbit coupling effect (SOC), PBE+vdW functional can reproduce the experimental electronic bandgap of hybrid halide perovskites well, in which the calculated electronic bandgap (∼2.13 eV, Figure ) of bulk PEA 2 PbI 4 is well consistent with experimental value (2.38 eV). The spin-orbit coupling effect (PBE+SOC), however, renders an electronic bandgap of PEA 2 PbI 4 (Table and Figure ) considerably smaller than the PBE-only one, thence SOC plays an indispensable role in the electronic structure of 2D monolayer PEA 2 MX 4 . Moreover, a strong correlation between PBE+SOC and HSE06+SOC level bandgap is found as a linear function with a scale factor of ∼1.25 ), which is consistent with the traditional wisdom. Bandgap values, similar to the lattice constants, correlate strongly with the atomic number of the halide elements (Figure ), due to the difference in the electronegativity and the ionic radii of X -anions.