Diamond cubic Si (Fd-3m), 8-atom conventional cell, a = 5.4310 A at ~300 K. Known-answer control structure for the Thermoelectrics API Seebeck route (measured S of doped Si spans 300-1000 K at 1e18-1e20 cm^-3, n- and p-type). Validation record (pymatgen + ASE, 2026-09-23): formula Si, 8 sites, density 2.3291 g/cm3 vs 2.329 handbook, space group Fd-3m recovered at symprec 0.01 and 0.1, min pair distance 2.352 A (sqrt(3)*a/4), full occupancies, round-trips through pymatgen and ASE. Written as P1 with explicit sites, same format as the corrected TiNiSn control.