A formulation of the Stillinger–Weber empirical potential is presented and employed for energetic calculations of various planar defects in GaN. The energies of inversion domain boundaries and of the I 1 stacking fault on the basal plane were calculated and compared to those of ab initio calculations from the literature. Present modification of empirical potential yielded comparable results with ab initio calculations. The potential was then applied for relaxation of large supercells comprising junction lines between inversion domain boundaries and stacking faults. The relaxed structures were used for HRTEM image simulations, which were compared with the corresponding experimental observations.