PLUTO: CRYSTAL PACKING ENERGIES IN THE CAMBRIDGE STRUCTURAL DATABASE. Sam Motherwell. Cambridge Crystallographic Data Centre, 12 Union Rd., Cambridge, CB2 lEZ. U.K.

The Pluto program has been enhanced to allow visual exploration of the packing energy environment of organic molecules. Facilities allow selection of the N strongest interactions. This allows comparison of structures on the basis of the packing energy enclosure shell about a molecule. The energy is calculated by the empirical atom-pair method, using the tables collected by Gavezzotti and others. This allows one to easily compare energy shells for molecules in polymorphs, and in general examine packing patterns on an energy basis. Selective display is allowed at a given energy threshold revealing the most important interactions in the packing. It is also possible to pick any pair of molecules in the packing diagram for energy display. Molecules may be simplified to molecular centre points, or spheres, or enclosure surfaces as ellipsoids or contact probed by a hydrogen atom. Coordinate information may be presented in a simple free format, or the standard Cambridge Structural Database FDAT. or SHELX.

Example. Benzene interactions < -1 Kcal/mole