Sequence of symbols associated with the Xdatcar. Similar to 6th line in
        vasp 5+ Xdatcar.

def site_symbols(self):
        """
        Sequence of symbols associated with the Xdatcar. Similar to 6th line in
        vasp 5+ Xdatcar.
        """
        syms = [site.specie.symbol for site in self.structures[0]]
        return [a[0] for a in itertools.groupby(syms)]