  We computer model a free-standing vitreous silica bilayer which has recently
been synthesized and characterized experimentally in landmark work. Here we
model the bilayer using a computer assembly procedure that starts from a single
layer of amorphous graphene, generated using a bond switching algorithm from an
initially crystalline graphene structure. Next each bond is decorated with an
oxygen atom and the carbon atoms are relabeled as silicon. This monolayer can
be now thought of as a two dimensional network of corner sharing triangles.
Next each triangle is made into a tetrahedron, by raising the silicon atom
above each triangle and adding an additional singly coordinated oxygen atom at
the apex. The final step is to mirror reflect this layer to form a second layer
and then attach the two layers together to form the bilayer.
  We show that this vitreous silica bilayer has the additional macroscopic
degrees of freedom to easily form a network of identical corner sharing
tetrahedra if there is a symmetry plane through the center of the bilayer going
through the layer of oxygen ions that join the upper and lower layers. This has
the consequence that the upper rings lie exactly above the lower rings, which
are tilted in general. The assumption of a network of perfect corner sharing
tetrahedra leads to a range of possible densities that we have previously
characterized in three dimensional zeolites as a flexibility window. Finally,
using a realistic potential, we have relaxed the bilayer to determine the
density, and other structural characteristics such as the Si-Si pair
distribution functions and the Si-O-Si bond angle distribution, which are
compared to the experimental results obtained by direct imaging.
