For example, if a non-crystallographic trimer were being refined with all monomers at the same weight, each monomer would be an equivalence set within a group. In this case, we would have one GROUp statement and three EQUIvalence selections, one for each monomer. If we wanted to restrain the side-chain atoms more loosely than the main chain, two GROUps would be given: the first would have 3 equivalence sets, each selecting a different monomer and the main chain atoms, with one set of weights; the second would also have three equivalence sets, each selecting a different monomer and the side-chain atoms, with a second set of weights. Unrestrained atoms are not selected in the EQUIvalence statement (e.g., those that are known to deviate due to lattice contacts).
A given atom can be used in more than one group, e.g., if it is involved in more than one internal symmetry relation. Each GROUp requires specification of the weights. This allows some restraints to be tighter than others.
Warning: No check is made to see if NCS-related atoms are frozen during
the refinement; all must be free, or all must be frozen.