This is the cutoff distance for Cα - Cα homology-derived restraints (it is passed to Restraints.make_distance() as the maximal_distance parameter when these restraints are generated). No Cα - Cα distances greater than this value will be used in building homology-derived restraints; thus, reducing this value from the default will typically reduce the number of restraints and increase the speed of optimization.
The default value of this parameter is 14.0 angstroms.
See also automodel.max_n_o_distance, automodel.max_n_o_distance, automodel.max_sc_mc_distance, and automodel.max_sc_sc_distance for similar cutoffs for the other kinds of homology-derived distance restraints.
See also automodel.very_fast(), which decreases all of these distances from their default values.