The adsorption of water on the Si(100) surface is investigated using ab initio quantum chemical cluster calculations. A reaction profile is constructed using the multiconfigurational SCF method. The calculations demonstrate that the reactant should be described with a multireference wave function in order to obtain correct energetics, because it contains a bare dimer with significant diradical character. The system becomes almost single-configurational as water approaches the surface and forms a molecularly adsorbed intermediate. Therefore, except for the reactant, a single-configurational wave function seems to be sufficient for a correct description of the reaction. The adsorbed OH group in an isolated product can nearly freely rotate between the trans and gauche minima. Interactions between the OH groups and the dangling bonds are small and do not appear to change the OH orientation. However, the interdimer hydrogen bonding is stronger and forces the OH orientation to be perpendicular to the dimer bond. The free rotation of the OH group in an isolated dimer model and the hydrogen-bonding picture in an extended cluster model are consistent with the experimental finding for the OH orientation in the product. Si9H12, Si15H16, Si32H28, Si48H36, and Si64H44 cluster models for the Si(100) surface are used, and the SIMOMM (surface integrated molecular orbital molecular mechanics) method is used effectively for these large cluster calculations. The SIMOMM and full quantum results are compared.