def get_bulk
      b = 0.25*self.lattice_const
      a1 = Atom.new(0, 0, 0, self.cation)
      a2 = Atom.new(b, b, b, self.anion)
      
      v1 = Vector[0.5, 0.5, 0.0]*self.lattice_const
      v2 = Vector[0.5, 0.0, 0.5]*self.lattice_const
      v3 = Vector[0.0, 0.5, 0.5]*self.lattice_const
      zb = Geometry.new([a1, a2], [v1, v2, v3])
      
      millerx = [1, 0, 0]
      millery = [0, 1, 0]
      millerz = [0, 0, 1]
      
      zb.set_miller_indices(millerx, millery, millerz)
      
      return zb
    end
def get_bulk
      b = 0.25*self.lattice_const
      a1 = Atom.new(0, 0, 0, self.cation)
      a2 = Atom.new(b, b, b, self.anion)
      
      v1 = Vector[0.5, 0.5, 0.0]*self.lattice_const
      v2 = Vector[0.5, 0.0, 0.5]*self.lattice_const
      v3 = Vector[0.0, 0.5, 0.5]*self.lattice_const
      zb = Geometry.new([a1, a2], [v1, v2, v3])
      
      millerx = [1, 0, 0]
      millery = [0, 1, 0]
      millerz = [0, 0, 1]
      
      zb.set_miller_indices(millerx, millery, millerz)
      
      return zb
    end
Initialize the zinc-blende Geometry
 cation and anion are the  atomic
 species occupying the two different sub-lattices.
 lattice_const specifies the lattice constant
 Return the traditional unit cell of bulk zinc blende