Reads a zmat file.

        Lines beginning with ``#`` are ignored.

        Args:
            inputfile (str):
            implicit_index (bool): If this option is true the first column
            has to be the element symbols for the atoms.
                The row number is used to determine the index.

        Returns:
            Zmat:

def read_zmat(cls, inputfile, implicit_index=True):
        """Reads a zmat file.

        Lines beginning with ``#`` are ignored.

        Args:
            inputfile (str):
            implicit_index (bool): If this option is true the first column
            has to be the element symbols for the atoms.
                The row number is used to determine the index.

        Returns:
            Zmat:
        """
        cols = ['atom', 'b', 'bond', 'a', 'angle', 'd', 'dihedral']
        if implicit_index:
            zmat_frame = pd.read_table(inputfile, comment='#',
                                       delim_whitespace=True,
                                       names=cols)
            zmat_frame.index = range(1, len(zmat_frame) + 1)
        else:
            zmat_frame = pd.read_table(inputfile, comment='#',
                                       delim_whitespace=True,
                                       names=['temp_index'] + cols)
            zmat_frame.set_index('temp_index', drop=True, inplace=True)
            zmat_frame.index.name = None
        if pd.isnull(zmat_frame.iloc[0, 1]):
            zmat_values = [1.27, 127., 127.]
            zmat_refs = [constants.int_label[x] for x in
                         ['origin', 'e_z', 'e_x']]
            for row, i in enumerate(zmat_frame.index[:3]):
                cols = ['b', 'a', 'd']
                zmat_frame.loc[:, cols] = zmat_frame.loc[:, cols].astype('O')
                if row < 2:
                    zmat_frame.loc[i, cols[row:]] = zmat_refs[row:]
                    zmat_frame.loc[i, ['bond', 'angle', 'dihedral'][row:]
                                   ] = zmat_values[row:]
                else:
                    zmat_frame.loc[i, 'd'] = zmat_refs[2]
                    zmat_frame.loc[i, 'dihedral'] = zmat_values[2]

        elif zmat_frame.iloc[0, 1] in constants.int_label.keys():
            zmat_frame = zmat_frame.replace(
                {col: constants.int_label for col in ['b', 'a', 'd']})
        zmat_frame = cls._cast_correct_types(zmat_frame)
        try:
            Zmat = cls(zmat_frame)
        except InvalidReference:
            raise UndefinedCoordinateSystem(
                'Your zmatrix cannot be transformed to cartesian coordinates')
        return Zmat