Sustainable economic production of hydrogen from water and sunlight is an attractive goal. It requires an active electrocatalyst comprised of earth-abundant elements. Such a catalyst exists in nature, the [FeFe](H) cluster in the active site of the di-iron hydrogenase enzymes. To reach the required specific activity within an actual solar hydrogen-production system, the catalytically active site must, figuratively, be stripped from the enzyme, attached to a cathode or photocathode, and immersed in water. Thus modifications of the composition and structure of the cluster are to be found which allow for stable attachment to the electrode surface and for maintenance of its integrity and activity throughout the H(2)-producing cycle in an environment drastically different from that in the enzyme. We have addressed that problem by simulating the behavior of model clusters by first-principles electronic-structure and molecular-dynamics simulations. We review our studies, first of the [FeFe](H) cluster in vacuum; next of the [FeFe](H) cluster in water; then of a systematic sequence of modifications which culminates with the design of the successful phosphorous-substituted [FeFe](P) cluster; and, finally, an investigation of the H(2) producing cycle of [FeFe](P). We then discuss the limitations of our results and conclude with a brief consideration of future directions. (c) 2010 Elsevier B.V. All rights reserved.