Density functional calculations are used to describe the nitrogenase FeMo cofactor and its interaction with N-2, H-2, H2O, and CO. Different models of the active part of the cofactor are investigated, and it is shown that a detailed description of the adsorption of different substrates can be obtained in good agreement with available experimental information. In addition, we propose a reaction path for the catalytic synthesis of ammonia through the gradual hydrogenation of molecular N-2 adsorbed on the Fe part of the cofactor.