Low-valency gallium PAW for faster defect calculations in GaN using plane wave DFT

Using an automated optimization scheme, we generate a gallium projector augmented wave (PAW) data set with the 3d electrons treated as core electrons, in contrast to typical treatment of the 3d electrons as valence electrons. Treating the 3d electrons as core electrons reduces calculation sizes, enabling faster defect studies of GaN. We show that the results of defect calculations in GaN are not adversely affected by freezing the 3d electrons to create the new PAW. For all three point defects tested (gallium vacancy, gallium interstitial in the tetrahedral site, and gallium interstitial in the octohedral site), the defect energy levels do not deviate by more than 0.25 eV from results with 3d electrons treated as valence. We show speedups in the defect calculations using the new PAW from 1.3 to 2.9, with increasing speedup for larger supercell size. These speedups would translate to similar speedups in Born-Oppenheimer molecular dynamics studies of GaN defects. In addition, we share some insight into the development of the optimized PAW including the use of the arctangent of the logderivatives-a superior accuracy metric for automated optimization of logderivatives/scattering properties.