Expressing ambiguous restraints using the optional OR statement

If a restraint is created with the ASSIgn ... OR ... syntax, the restraint's average distance is calculated in a manner slightly modified from that detailed in Section 20.2. The benefit of expressing assignment ambiguity with the ASSIgn ... OR ... syntax is that only the relevant atom pairs for each possible assignment are included.

A restraint created with the ASSIgn ... OR ... syntax has multiple pairs of atom selections, each of which is evalulated during the calculation of the average distance. When the ASSIgn ... OR syntax is used, SUM averaging is forced on, no matter which specifier is given to the AVERage statement. The distance between selected sets of atoms is computed by adding up single contributions

$$R = ( \sum_{k} \sum_{i_kj_k} R_{i_kj_k}^{-6} / n_{mono} )^{-1/6}$$ (20.5)

where the sum over $k$ runs through each of pairs of atom selections, and the sum over $i_k$ and $j_k$ runs through every possible combination of atoms in the associated selection pair's first and second members. $n_{pairs}$ is the total number of pairs of atoms that the two sums evaluate and $n_{mono}$ is specified by the MONOmer statement.