We investigate possible structures of hydrogen-induced platelets in the (100) crystallographic plane in silicon using ab initio methods. We consider the structures of Si(100) external surfaces - the 2×1-reconstructed monohydride and the 1×1-reconstructed dihydride - as possible structures of hydrogen-induced platelets. We find that the 1×1 reconstructed dihydride-terminated structure has the lowest formation energy per hydrogen atom. Addition of H2 molecules to platelets makes the formation energies of platelets lower. We discuss the vacancy-based model of (100) platelet structures and compare the energies of (100) platelets with platelets in the (111) plane in silicon. Energies of hydrogen-induced platelets in (100) and (111) planes are found to be very similar. Therefore, we conclude that the preferred crystallographic orientation of platelets is not caused by thermodynamic stability but by kinetic reasons: wafer surface orientation and diffusion of hydrogen to platelets. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim