CRITICAL-POINT ANALYSIS OF CALCULATED ELECTRON-DENSITY MAPS AT MEDIUMRESOLUTION - APPLICATION TO SHAPE-ANALYSIS OF ZEOLITE-LIKE SYSTEMS

A method, derived from the procedure implemented in the program ORCRIT which was originally developed by C.K. Johnson, is presented for the topological analysis of zeolite electron density maps. This method is an approach to shape analysis of zeolitic frameworks through the locat ion, identification, and linkage of critical points (points where the gradient of the density vanishes) in medium resolution electron densit y maps. It is observed that, at 3 Angstrom resolution, tetrahedral sit es and oxygen atoms generate peaks (density maxima) and passes (saddle points), respectively. The defnition of ellipsoids, built at each pea k location from their three local density curvatures, is used to gener ate a void volume wherein atomic or molecular probes can interact. The energetic behaviour of probes of several sizes interacting with ferri erite and mordenite is qualitatively studied through a pseudo- Lennard -Jones potential approximation.