def displ_iter(self, g_nums, ats_1, ats_2, invalid_error=False):
        """ Iterator over indicated displacement vectors.

        Displacements are in Bohrs as with :meth:`displ_single`.

        See `above <toc-generators_>`_ for more information on
        calling options.

        Parameters
        ----------
        g_nums
            |int| or length-R iterable |int| or |None| --
            Index/indices of the desired geometry/geometries

        ats_1
            |int| or length-R iterable |int| or |None| --
            Index/indices of the first atom(s)

        ats_2
            |int| or length-R iterable |int| or |None| --
            Index/indices of the second atom(s)

        invalid_error
            |bool|, optional --
            If |False| (the default), |None| values are returned for
            results corresponding to invalid indices. If |True|,
            exceptions are raised per normal.

        Yields
        ------
        displ
            |npfloat_| --
            Displacement vector in Bohrs between each atom pair of |br|
            `ats_1` :math:`\\rightarrow` `ats_2` from the corresponding
            geometries of `g_nums`.

        Raises
        ------
        ~exceptions.IndexError
            If an invalid (out-of-range) `g_num` or `at_#` is provided.

        ~exceptions.ValueError
            If all iterable objects are not the same length.

        """

        # Import the tuple-generating function
        from .utils import pack_tups

        # Print the function inputs if debug mode is on
        if _DEBUG:  # pragma: no cover
            print("g_nums = {0}".format(g_nums))
            print("ats_1 = {0}".format(ats_1))
            print("ats_2 = {0}".format(ats_2))
        ## end if

        # Perform the None substitution
        arglist = self._none_subst(g_nums, ats_1, ats_2)

        # Expand/pack the tuples from the inputs
        tups = pack_tups(*arglist)

        # Dump the results if debug mode is on
        if _DEBUG:  # pragma: no cover
            print(tups)
        ## end if

        # Construct the generator using the packed tuples.
        for tup in tups:
            yield self._iter_return(tup, self.displ_single, invalid_error)