A layer of iodine at Ge(001) surface develops an ordered structure of iodine atoms bound to Ge dimers. Here are discussed atomic structures of Ge(001) surface covered by 0.25 monolayer of iodine. The p(2 x 4), p(2 x 2), c(2 x 4) and p(1 x 4) surface structures are found in calculations. The structure with two iodine atoms of the dissociated I-2 molecule adsorbed at both ends of the same germanium dimer is found to be energetically favourable over iodine adsorption at neighbouring dimers. Simulated STM images of the obtained surface structures are presented and compared with experimental data. (C) 2008 Elsevier B.V. All rights reserved.