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NCS Restraints

A ``group" refers to a set of equivalent atoms. The GROUp statement tells the program that a new set is being input. The EQUIvalence selection serially defines the NCS-related atoms within the group. That is, the first atoms of each equivalence set are related by NCS, the second atoms of each set are related by NCS, etc.; the program superimposes the selected atoms by their position in the equivalence set. X-PLOR demands that each equivalence selection within a group have the same number of atoms, residue names, and atom names; thus NCS-related atoms can be distinguished by segment name or residue number (typically, separate chains are given different segment names).

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.

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