Ge diamond known-geometry control (exp a=5.6579 A, Fd-3m primitive)

Known-geometry control for the DFT relax route (ABACUS). Ge diamond primitive cell at experimental a=5.6579 A. Reference correction (2026-09-13): plain PBE relaxes Ge to a~5.75-5.77 A (the known ~+2% PBE Ge overestimate); an earlier version of this description quoted 5.66-5.71 A, too narrow. The 2026-09-12 control run relaxed to a_conv=5.7768 A (+0.15% vs PBE literature 5.768) - pass. Route settings matched to the MnAlGe contract run: PBE, DZP, ecutwfc 65, kspacing 0.16, scf_thr 1e-6, mp smearing 0.05 eV, broyden 0.4, no U.