Acknowledgments

We are grateful to F. Cacialli, E. Marseglia, R. H. Friend, R. J. Needs and E. Tedesco (Cavendish Laboratory, Cambridge) for many inspiring discussions. The calculations have been obtained with the PARATEC [57] and ABINIT [58] codes. The computational resources were provided by the University of Cambridge High Performance Computing Facility. We acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada (M.C.), Magdalene College, Cambridge (P.D.H.), and of the UK Engineering and Physical Sciences Research Council (C.M.).