The bond order according the distance between the two sites
        Args:
            tol (float): Relative tolerance to test.
                (1 + tol) * the longest bond distance is considered
                to be the threshold length for a bond to exist.
                (1 - tol) * the shortest bond distance is considered
                to be the shortest possible bond length
                Defaults to 0.2.
            default_bl: If a particular type of bond does not exist,
                use this bond length as a default value
                (bond order = 1). If None, a ValueError will be thrown.
        Returns:
            Float value of bond order. For example, for C-C bond in
            benzene, return 1.7.

def get_bond_order(self, tol=0.2, default_bl=None):
        """
        The bond order according the distance between the two sites
        Args:
            tol (float): Relative tolerance to test.
                (1 + tol) * the longest bond distance is considered
                to be the threshold length for a bond to exist.
                (1 - tol) * the shortest bond distance is considered
                to be the shortest possible bond length
                Defaults to 0.2.
            default_bl: If a particular type of bond does not exist,
                use this bond length as a default value
                (bond order = 1). If None, a ValueError will be thrown.
        Returns:
            Float value of bond order. For example, for C-C bond in
            benzene, return 1.7.
        """
        sp1 = list(self.site1.species.keys())[0]
        sp2 = list(self.site2.species.keys())[0]
        dist = self.site1.distance(self.site2)
        return get_bond_order(sp1, sp2, dist, tol, default_bl)