Calculations of free energy barriers for local mechanisms of hydrogen diffusion in alanates

Brute force histogram calculation and a recently developed method to efficiently reconstruct the free energy profile of complex systems ( the single-sweep method) are combined with ab initio molecular dynamics to study possible local mechanisms for the diffusion of hydrogen in sodium alanates. These compounds may help to understand key properties of solid state hydrogen storage materials. In this work, the identity of a mobile species observed in experiments characterizing the first dissociation reaction of sodium alanates is investigated. The activation barrier of two suggested processes for hydrogen diffusion in Na(3)AlH(6) is evaluated and, by comparing our results with available experimental information, we are able to discriminate among them and to show that one is compatible with the observed signal while the other is not.