Gives the transformation matrix to transform a conventional
        unit cell to a primitive cell according to certain standards
        the standards are defined in Setyawan, W., & Curtarolo, S. (2010).
        High-throughput electronic band structure calculations:
        Challenges and tools. Computational Materials Science,
        49(2), 299-312. doi:10.1016/j.commatsci.2010.05.010

        Returns:
            Transformation matrix to go from conventional to primitive cell

def get_conventional_to_primitive_transformation_matrix(self, international_monoclinic=True):
        """
        Gives the transformation matrix to transform a conventional
        unit cell to a primitive cell according to certain standards
        the standards are defined in Setyawan, W., & Curtarolo, S. (2010).
        High-throughput electronic band structure calculations:
        Challenges and tools. Computational Materials Science,
        49(2), 299-312. doi:10.1016/j.commatsci.2010.05.010

        Returns:
            Transformation matrix to go from conventional to primitive cell
        """
        conv = self.get_conventional_standard_structure(
            international_monoclinic=international_monoclinic)
        lattice = self.get_lattice_type()

        if "P" in self.get_space_group_symbol() or lattice == "hexagonal":
            return np.eye(3)

        if lattice == "rhombohedral":
            # check if the conventional representation is hexagonal or
            # rhombohedral
            lengths, angles = conv.lattice.lengths_and_angles
            if abs(lengths[0]-lengths[2]) < 0.0001:
                transf = np.eye
            else:
                transf = np.array([[-1, 1, 1], [2, 1, 1], [-1, -2, 1]],
                                  dtype=np.float) / 3

        elif "I" in self.get_space_group_symbol():
            transf = np.array([[-1, 1, 1], [1, -1, 1], [1, 1, -1]],
                              dtype=np.float) / 2
        elif "F" in self.get_space_group_symbol():
            transf = np.array([[0, 1, 1], [1, 0, 1], [1, 1, 0]],
                              dtype=np.float) / 2
        elif "C" in self.get_space_group_symbol() or "A" in self.get_space_group_symbol():
            if self.get_crystal_system() == "monoclinic":
                transf = np.array([[1, 1, 0], [-1, 1, 0], [0, 0, 2]],
                                  dtype=np.float) / 2
            else:
                transf = np.array([[1, -1, 0], [1, 1, 0], [0, 0, 2]],
                                  dtype=np.float) / 2
        else:
            transf = np.eye(3)

        return transf