Inversion symmetry operation about axis.

        Args:
            origin (3x1 array): Origin of the inversion operation. Defaults
                to [0, 0, 0].

        Returns:
            SymmOp representing an inversion operation about the origin.

def inversion(origin=(0, 0, 0)):
        """
        Inversion symmetry operation about axis.

        Args:
            origin (3x1 array): Origin of the inversion operation. Defaults
                to [0, 0, 0].

        Returns:
            SymmOp representing an inversion operation about the origin.
        """
        mat = -np.eye(4)
        mat[3, 3] = 1
        mat[0:3, 3] = 2 * np.array(origin)
        return SymmOp(mat)