code: def to_string(self):
          
          lines = []
          app = lines.append
  
          app("CRYSTAL")
          app("# Primitive lattice vectors in Angstrom")
          app("PRIMVEC")
          cell = self.structure.lattice.matrix
          for i in range(3):
              app(' %.14f %.14f %.14f' % tuple(cell[i]))
  
          cart_coords = self.structure.cart_coords
          app("# Cartesian coordinates in Angstrom.")
          app("PRIMCOORD")
          app(" %d 1" % len(cart_coords))
  
          for a in range(len(cart_coords)):
              sp = "%d" % self.structure.atomic_numbers[a]
              app(sp + ' %20.14f %20.14f %20.14f' % tuple(cart_coords[a]))
  
          return "\n".join(lines)
docs: Returns a string with the structure in XSF format
          See http://www.xcrysden.org/doc/XSF.html