The potential energy surfaces for the reactions of silicon atom (ID and 3P) with the water molecule were calculated by ab initio self-consistent field (SCF) methods. The transition states for the insertion of silicon atom into the D--H bond of water were also calculated by the multiconfigurational SCF method. It is found that the crossing point of the singlet and triplet surfaces along the insertion reaction paths is near the transition state for the triplet rearrangement from the Si:OH2 complex to HSiOH. The potential energy surfaces for the 1,2-hydrogen migration HSiOH- H2Si0 in the singlet and triplet states and the hydrogen (H2) elimination from HSiOH and H2Si0 were also investigated.