The energetics and electronic behavior of SiO2 . BeF2, based on interpenetrating diamondoid nets and recently proposed as a possible hard material, have been studied with ab initio periodic LCAO Hartree-Fock methods. The composite compound turns out to be unstable with respect to the precursors SiO2 and BeF2. The origin of the instability has been analyzed with the aid of crystal orbital displacement (COD) plots. A bulk modulus close to the sum of those of the two precursors is calculated for SiO2 . BeF2. The additivity of the bulk moduli of subnets suggests that hard precursors should be required to obtain superhard materials.