The adsorption of CH3NH2 on Si(0 0 1)-2 x 1 is investigated using density functional theory slab calculations. When the CH3NH2 coverage is greater than 0.5 ML, we find that the adsorption energetics alter significantly due to a combination of. (i) adsorbate-adsorbate repulsive interactions, (ii) adsorbate-induced electronic structure changes of neighboring dimers, and (iii) hydrogen-bonding interactions. Our calculations show that, among them, adsorbate-adsorbate repulsion plays a major role in determining the two-dimensional ordering of CH3NH2 adsorption at the full coverage of 1.0 ML, leading the zigzag structure to be energetically more favorable than the aligned structure along the dimer row. (C) 2004 Published by Elsevier B.V.