def faceGrad(self): r""" Return :math:`\nabla \phi` as a rank-1 `FaceVariable` using differencing for the normal direction(second-order gradient). """ if not hasattr(self, '_faceGrad'): from fipy.variables.faceGradVariable import _FaceGradVariable self._faceGrad = _FaceGradVariable(self) return self._faceGrad