Dear friends, I'm having trouble understanding an article by M. Cohen and J. Corkill (Structural, bonding, and electronic properties of IIA-IV antifluorite compounds). In this article Be2Si (an imaginary compound) is shown to be a metal. The argument used is GROUP SYMMETRY :... group symmetry requirements produce a band along Delta that crosses the fermi level.... My question is: what are these group symmetry requirements? How do they know the Fermi level is there? I show you the band structure if you want to have a look. Thanks!