Architecture of packing in molecular crystals
Abstract
Several classification criteria based on the molecular coordination shell have been suggested to bring together crystal structures with similar packing architecture. A chart of the shell interactions sorted by energy shows a feature where there is an abrupt step in the curve, at the position Nstep. Comparison of structures with the same Nstep reveals architectural similarity. A second criterion involving the number of strong energy planes, Npl, within a chosen maximum deviation, also reveals similarity in structures. A third useful criterion for comparison is the projection pattern, Npatt, formed by projecting the coordination shell into 2D. The projection pattern, Npatt, is characterised by a symmetry code, Pcode, giving insight into common architectural patterns shared by many structures. A survey of the CSD shows a surprising number of structures having identical patterns. While these criteria do not guarantee close geometric 3D similarity of the coordination shell molecular centres, they do reveal similar symmetry and geometry of packing.