def fc2(self):
        files = shell_exec("ls dirs").split('\n')
        files = map(lambda x: x.replace('dir_', ''), files)
        # when the number of files >1000, the order is wrong ,POSCAR-001,
        # POSCAR-1500 ,POSCAR-159
        files.sort(lambda x, y: int(x.split('-')[1]) - int(y.split('-')[1]))
        self.force_constant(files)