Chemical units may extend over more than one asymmetric unit. They may have a symmetry element that coincides with the space group symmetry such as an inversion centre or a screw axis. In such cases we will find atoms in the above search for a connected set of atoms that are bonded to the connected set at a position different from the one that was fixed in view of an earlier connection. Those atoms are added to the connected set and marked as symmetry related. The symmetry operation of this atom with respect to the primary one is coded and added to the molecule (aru) list. A chemical unit around an inversion centre thus consists in the PLUTO78 terminology of two molecules: 1555 and 2555.
A further complication may be the presence of more than one crystallographically independent chemical unit in the unit cell (including solvent molecules and anions). In that case not all atoms will be fixed when the above procedure comes to an end when no more connected atoms are found. In that case a new residue is started by again arbitrarily fixing a suitable atom and expanding it to a connected set.
A particular residue r within a molecule is indicated with the code sklm.r (e.g. 3564.03). It is understood that the code without a fraction stands for the full collection of residues. Thus in the case of two residues the molecule code 2562 is equivalent with the two residue codes 2562.01 and 2562.02. In order to be more precise two new terms have been introduced in PLUTON and PLATON. The basic structural unit is the asymmetric residue unit ( = aru ) coded as sklm.r. A molecule (ion) will will be an assembly of at least one aru. The set of aru's making up the asymmetric structural unit are called asu and encoded as sklm.
Based on the supplied population parameters attemps are made to suppress bonds between disorder parts with unequal population parameter values.