Although X-PLOR provides improper angles to maintain planarity of groups of atoms, the specification of these energy terms is sometimes cumbersome, especially when many atoms are involved. The restraints planarity statement allows one to define an effective energy term that penalizes out-of-plane conformations of the selected atoms. This feature should be used for relatively few groups, as it involves a certain amount of overhead. The planarity restraints energy term is defined as
where the first sum is carried out over all defined groups of planar atoms, the second sum is carried out for all atoms i within each group, and is the orthogonal distance of i from the least-squares plane defined by all atoms of the group (Schomaker et al. 1959). Note that the energy term must be activated through the flags statement (Section 4.5).