ABSEN - A PC computer program for listing systematic absences and space-group determination

Abstract

The crystallographic problem: Spacegroup determination from systematic absences is one of the most important as-pects of crystallography. It is important to know both just how many reflections determine a particular condition and all possible space groups that satisfy observed conditions. ABSEN makes this process systematic. Method of solution: ABSEN is a computer program that lists reflections into convenient groups and displays possible extinctions at three cut-off levels. The observed extinctions are used to predict possible space groups in all orientations. The program reads a reflection file, *.HKL, in either SHELX format (314, 2F8.2) or free delimited format. Each reflection is read as H, K, L, X and σX, where X may be F or F2. If defaults are selected, the screen output is as in Table 1. If defaults are not selected, the cut-off levels, normally at X/σX of 3, 6 and 12, may be adjusted and a complete extinction list is written to the screen. There is also an output file that contains the full table and either all axial reflections or a single set of reflections. The group of reflections that cause any condition to fail at the highest cut-off level may also be written to output. If hexagonal settings are used, then the default output file contains the 0001 reflections, written as hkl (hkil) XσX. The other nongeneral hexagonal reflection sets may also be written to the output file. A particularly useful feature is the listing in column 5 (odd) of the precise number of reflections on which the condition depends. When conditions are present at the highest cut-off level, the corresponding operations are moved to column 9. All space groups (in all orientations) that correspond to the conditions in column 9 are listed in Table 1. Nonstandard orthorhombic settings are detected and a transformation matrix to the standard setting is generated. The nonstandard monoclinic settings which cannot be avoided at the indexation/transformation stage are included explicitly. Hardware environment: The program requires at least a 386/387, DOS 5 and 4 Mbytes of RAM. Availability: The program and test data are available via FTP or on 1.44 Mbyte floppy disk from the author p.mcardle@ucg.ie. © 1996 International Union of Crystallography.