Six complexes between a model active site of alpha-chymotrypsin (261 atoms) and N-acetyl-L-tryptophanamide (33 atoms) were optimized at the semiempirical AM1 level. In one of these complexes, a water molecule was included. A detailed study at the geometric and energetic levels is presented. The discussion deals with the nature of the interaction, the effect of the environment, the ligand deformation, the backbone relaxation, the water molecule freedom, and the comparison with the molecular mechanics results.