Bilbao Crystallographic Server Structure Utilities Help COMPSTRU

Comparison of crystal structure descriptions [COMPSTRU]

Often a quantitative comparison of two structure descriptions of the same phase, coming from different sources, is difficult because the descriptions, although postulating the same space group symmetry, are described using different sets of atoms in the asymmetric unit, or different equivalent choices of origin or cell orientations. The program COMPSTRU compares two structure descriptions of the same or different composition using the following algorithm:

First, the program transforms the description of Structure 2 to the most similar configuration of the description of Structure 1 (the so-called reference description). After that the description of Structure 2 in the most similar configuration to the reference description is chosen so that the maximal difference d_max of the atomic positions of the matching atoms has a minimal value. The difference between the two models is quantified by evaluation of the global distortion decomposed into a spontaneous strain (lattice deformation) and atomic displacement field representing the differences in the positions of the matching atoms of the two structure descriptions.

The determination of the transformation matrix that transforms the description of Structure 2 to the most similar configuration to the reference description is done in two steps:

1. If originally the two structure descriptions are referred to settings (although standard) that differ in the choice of the unit cell (with axes interchanged in the orthorhombic case or more general cell transformations in the case of monoclinic or triclinic structures), the program searches for a transformation that maps best the lattice parameters of the description of Structure 2 to those of the description of Structure 1. The transformation should belong to the affine normalizer of the structure space group.


2. The transformation of the modified description of Structure 2 to the most similar configuration to the reference description is determined so that the maximal difference d_max of the atomic positions of the matching atoms has a minimum value. The program uses the Euclidean normalizer of the common space group for exploring all possible equivalent descriptions of the description of Structure 2 as modified in Step (i).

The input structural data should be described with respect to the standard (default) settings of the structure space group. If transformation of the structure descriptions into standard settings is necessary, then the program SETSTRU or TRANSTRU on this server can be used.

The program COMPSTRU can also be used for the recognition of similar atomic arrangements of different compounds that is essential for the classification of crystal structures into isoconfigurational structure types.