Using a first-principles approach, we investigate the effect of interdimer interactions on N Is core-level shifts at the NH3 saturated Si(100)-2 X 1 surface. Fully relaxed model structures are generated in which -NH2 groups on adjacent dimers in a row are located either on opposite ends (OE) or on the same ends of the respective dimers. By comparing calculated core-level shifts with measured photoemission spectra, we show that the OE configuration dominates, supporting the occurrence of an ordered coverage pattern. (C) 2001 Elsevier Science B.V. All rights reserved.