def delta(aCell)
      raise "Cells do not have the same number of atoms" unless self.atoms.size == aCell.atoms.size

      pseudo_atoms = []
      self.atoms.size.times {|i|
        a1 = self.atoms[i]
        a2 = aCell.atoms[i]
        raise "Species do not match" unless a1.species == a2.species
        a = Atom.new
        a.species = a1.species
        a.x = a1.x - a2.x
        a.y = a1.y - a2.y
        a.z = a1.z - a2.z
        pseudo_atoms << a
      }
      Geometry.new(pseudo_atoms)
    end
def delta(aCell)
      raise "Cells do not have the same number of atoms" unless self.atoms.size == aCell.atoms.size

      pseudo_atoms = []
      self.atoms.size.times {|i|
        a1 = self.atoms[i]
        a2 = aCell.atoms[i]
        raise "Species do not match" unless a1.species == a2.species
        a = Atom.new
        a.species = a1.species
        a.x = a1.x - a2.x
        a.y = a1.y - a2.y
        a.z = a1.z - a2.z
        pseudo_atoms << a
      }
      Geometry.new(pseudo_atoms)
    end
Find the difference between this cell and another cell
 Return a cell with Pseudo-Atoms whose positions are really the differences