Sequence of number of sites of each type associated with the Poscar.
        Similar to 7th line in vasp 5+ Xdatcar.

def natoms(self):
        """
        Sequence of number of sites of each type associated with the Poscar.
        Similar to 7th line in vasp 5+ Xdatcar.
        """
        syms = [site.specie.symbol for site in self.structures[0]]
        return [len(tuple(a[1])) for a in itertools.groupby(syms)]