In this paper, we expand our previous paper [Mod. Phys. Lett. B 32 (2018) 1750355] from the calculation of planar graphene with one band (pz) to the calculation of two-dimensional buckled group-IVA materials with multiple bands (S, px, py, and pz); thus, the proposed method is a fullband model. Furthermore, the proposed method is established using a nonequilibrium Green's function (NEGF) method in association with the complex energy-band technique, so it is in the full-quantum framework. Unlike other methods, the proposed method is noniterative and thus computationally cost-efficient.