def displace(x,y,z)
      Geometry.new(atoms(:all).collect{|a|
        a.displace(x,y,z)
      }, self.lattice_vectors)
      #TODO copy miller indices
    end
def displace(x,y,z)
      Geometry.new(atoms(:all).collect{|a|
        a.displace(x,y,z)
      }, self.lattice_vectors)
      #TODO copy miller indices
    end
Return a new unit cell with all the atoms displaced by the amount x,y,z