Computer simulation of crystal structures

It is impossible to calculate from first principles the details of crystal structures. However, it is possible to rely on information which is certain, namely on interatomic distances, and calculate the details of a crystal structure based on these. Individual distances between atoms can be predicted accurately if one considers the coordination numbers of anions and cations, the extended electrostatic valence rule, and the effects of shared edges between different coordination polyhedra. The predicted interatomic distances are used as observations in a distance least squares refinement in which the positional parameters and the cell edges are adjusted until the calculated interatomic distances correspond as closely as possible to the predicted distances. The topology of the simulated structure has to be known or assumed. Appropriate weighting can facilitate proper modelling. Applications include: a) solution of pseudosymmetric structures; b) preliminary refinement of trial structures; c) geometric refinement without X-ray intensities; d) comparison of hypothetical structures with observed polymorphs of the same compositions; e) simulation of temperature dependence of structures; f) simulation of pressure dependence; g) calculation of structures isomorphous to known structures; h) calculation of thermal ellipsoids; i) calculation of local environments deviating from the overall symmetry of a structure; j) testing of hypotheses about the behavior of structures at varying conditions.