Finally, Figure examines the "second-order" bond restraints that act on the distances between pairs of assigned atoms. Turning these restraints off has little or no effect for the assembly of smaller targets, but we find that a restraint strength of at least 0.5 is needed to reliably assemble targets of 20 atoms or more. However, we also find that these restraints can hinder assembly of some smaller targets, particularly molecule 10, which has high symmetry. We also have observed some numerical instabilities in simulations with bond restraint values > 1.0. For this reason we see a value of 0.5 to be a good compromise.