We would like to thank F. Cacialli, E. Marseglia, R. H. Friend, R. J. Needs and E. Tedesco from the Cavendish Laboratory for many useful discussions. The computational resources were provided by the University of Cambridge High Performance Computing Facility. We acknowledge the support of the Natural Sciences and Engineering Research Council of Canada (M.C.), Magdalene College, Cambridge (P.D.H.), and of the U.K. Engineering and Physical Sciences Research Council (P.D.H. and C.M.).