def break_bond(self, ind1, ind2, tol=0.2):
        """
        Returns two molecules based on breaking the bond between atoms at index
        ind1 and ind2.

        Args:
            ind1 (int): Index of first site.
            ind2 (int): Index of second site.
            tol (float): Relative tolerance to test. Basically, the code
                checks if the distance between the sites is less than (1 +
                tol) * typical bond distances. Defaults to 0.2, i.e.,
                20% longer.

        Returns:
            Two Molecule objects representing the two clusters formed from
            breaking the bond.
        """
        sites = self._sites
        clusters = [[sites[ind1]], [sites[ind2]]]

        sites = [site for i, site in enumerate(sites) if i not in (ind1, ind2)]

        def belongs_to_cluster(site, cluster):
            for test_site in cluster:
                if CovalentBond.is_bonded(site, test_site, tol=tol):
                    return True
            return False

        while len(sites) > 0:
            unmatched = []
            for site in sites:
                for cluster in clusters:
                    if belongs_to_cluster(site, cluster):
                        cluster.append(site)
                        break
                else:
                    unmatched.append(site)

            if len(unmatched) == len(sites):
                raise ValueError("Not all sites are matched!")
            sites = unmatched

        return (self.__class__.from_sites(cluster)
                for cluster in clusters)
Returns two molecules based on breaking the bond between atoms at index
        ind1 and ind2.

        Args:
            ind1 (int): Index of first site.
            ind2 (int): Index of second site.
            tol (float): Relative tolerance to test. Basically, the code
                checks if the distance between the sites is less than (1 +
                tol) * typical bond distances. Defaults to 0.2, i.e.,
                20% longer.

        Returns:
            Two Molecule objects representing the two clusters formed from
            breaking the bond.