def _write_pair_information(gsd_file, structure):
    """Write the special pairs in the system.

        Parameters
    ----------
    gsd_file :
        The file object of the GSD file being written
    structure : parmed.Structure
        Parmed structure object holding system information
    """
    pair_types = []
    pair_typeid = []
    pairs = []
    for ai in structure.atoms:
        for aj in ai.dihedral_partners:
            #make sure we don't double add
            if ai.idx > aj.idx:
                ps = '-'.join(sorted([ai.type, aj.type], key=natural_sort))
                if ps not in pair_types:
                    pair_types.append(ps)
                pair_typeid.append(pair_types.index(ps))
                pairs.append((ai.idx, aj.idx))
    gsd_file.pairs.types = pair_types
    gsd_file.pairs.typeid = pair_typeid
    gsd_file.pairs.group = pairs
    gsd_file.pairs.N = len(pairs)
Write the special pairs in the system.

        Parameters
    ----------
    gsd_file :
        The file object of the GSD file being written
    structure : parmed.Structure
        Parmed structure object holding system information