hi, For some reason iterating over restraints like: for(Restraints::iterator it = opt_mdl->get_restraints().begin(); it != opt_mdl->get_restraints().end(); it++) { } fails as *it == NULL, even though it contains data. The problem is solved if iterating like: for(unsigned int i =0;i<opt_mdl->get_restraints().size();i++){} Keren.