The mechanism for H₂ activation catalyzed by [NiFe] hydrogenases is investigated with a series of models for the Ni(II) and Ni(III) forms in both high-spin (HS) and low-spin (LS) states by density functional theory (DFT/B3LYP) calculations. The geometry optimizations include unconstrained models, partially constrained (to the crystal structure parameters) models and models with addition of nearby protein residues. Several uncertainties concerning the mechanism are addressed in our study: (1) the oxidation state of the active species that binds and cleaves H₂; (2) the structures and spin states prevalent in active site forms; (3) the influence of the surrounding protein environments on the active site. Adding the nearby protein residues to a fairly rigid active site framework stablizes the LS Ni(II) species. Although models for Ni–SI forms, with a vacant binding site, still prefer HS, addition of H₂ or CO stablizes the LS form. Thus, access to this LS state and two-state reactivity may play a role in the mechanism. Furthermore, the more complete protein models show that the energetic preference for the binding site for both H₂ and CO changes from Fe to Ni. This change brings the computational results in closer accord with the experimental ones.