H2 anchor structure (rare-earth-free magnet program): L1_0-ordered FeNi (tetrataenite prototype), P4/mmm #123, 2 atoms (Fe 0,0,0; Ni 1/2,1/2,1/2), a=2.516 A, c=3.559 A. Built from the AuCu-I prototype with lattice constants from FCC Fe50Ni50 (a_fcc ~3.559 A); validated 2026-09-05: parses, SG 123 at symprec 0.01 and 0.1, rho 8.44 g/cm3 (exp ~8.2-8.3), dmin 2.516 A, ordered.