def _approximate_unkown_bond_lengths(self):
dataset = self.lengths[BOND_SINGLE]<EOL>for n1 in periodic.iter_numbers():<EOL><INDENT>for n2 in periodic.iter_numbers():<EOL><INDENT>if n1 <= n2:<EOL><INDENT>pair = frozenset([n1, n2])<EOL>atom1 = periodic[n1]<EOL>atom2 = periodic[n2]<EOL>if (pair not in dataset) and (atom1.covalent_radius is not None) and (atom2.covalent_radius is not None):<EOL><INDENT>dataset[pair] = (atom1.covalent_radius + atom2.covalent_radius)<EOL><DEDENT><DEDENT><DEDENT><DEDENT>
Completes the bond length database with approximations based on VDW radii