DERIVATION OF CONVENTIONAL CRYSTALLOGRAPHIC DESCRIPTIONS OF NEW PHASES FROM RESULTS OF AB-INITIO INORGANIC STRUCTURE MODELING

The results of ab initio inorganic structure modelling are often in th e form of Cartesian coordinates of atoms in a large, periodical and in general oblique simulation box containing hundreds to thousands of at oms. The contents of that box may correspond to a single crystal, a tw in, a mixture of phases or a disordered block of matter. The problem o f extracting corresponding crystallographic descriptions for single-cr ystal regions in the box, a necessary step in view of full quantum cal culations and publication, is different from the familiar problem of e xtracting crystal symmetry and structure from experimental diffracted intensity data. The deductive computer-aided method developed at the N ational Research Council of Canada over the years is based on eye iden tification of three pairs of atoms related by conjugate translations i n the same single-crystal region on a stereo plot, followed by derivat ion of fractional coordinates for the atomic content of the correspond ing primitive cell. Running this data through the MISSYM program discl oses potential symmetry elements of the structure, with their correspo nding crystallographic directions. These elements are then critically examined and accepted either as symmetry or pseudosymmetry on the basi s of comparison of coordinate deviations between related atoms with th e expected magnitude of thermal motion. All calculations described her e can be performed with the NRCVAX system of programs.