We present a theoretical study of chemisorption of CH C-CH2-COOH molecules on the H:Si(100) surface. We perform simulations for different chemisorbed configurations, attained by reactions through the alkyne tail. We use the periodic slab approximation for the extended surface, within ab initio density functional theory, and analyse results from several different approaches. We conclude that structures composed of single Si-C bridges are very stable.. while a previously proposed structure, with a double Si-C-Si bridge, should be metastable on the flat surface, and introduce electron and hole traps in the Si band gap. (c) 2006 Elsevier B.V. All rights reserved.