Acknowledgements

The work of the FOM Institute is part of the research program of `Stichting Fundamenteel Onderzoek der Materie' (FOM) and is supported by NWO (`Nederlandse Organisatie voor Wetenschappelijk Onderzoek'). JPKD acknowledges the financial support provided by the Computational Materials Science program of the NWO, and RPS would like to thank The Royal Society for the award of a fellowship. We also thank Mark Miller for use of a program to perform the multi-histogram analysis.