Chapter 1. Preface

In most respects the structure, and the presentation, of the Gnu Xtal System[1] of Crystallographic Programs is little different to that of any recent versions. Previous users of this system will find that standard calculations are largely unchanged, though perhaps with the exception of STARTX.

In fact little change can be expected in the future. With the termination of actively funded maintenance any future advances of the suite lie in the hands of any volunteers with a penchant for hacking RATMAC/fortran77/C/tcl/tk code.

Xtal was originally developed with a goal toward being as versatile and comprehensive a suite of crystallographic software as was realistically possible. The cessation of actively funded development has dictated a more concessionary approach towards complementary software. To that end the program SHELIN has been modified slightly to read supplementary SHELX .ins files[2] and DIFDAT has been modified to read SHELX fixed format .hkl files (F^2 only). The program OUTSRC has been added to aid in creating VALRAY, XD and SHELX format input files from an Xtal archive. Those are probably the most useful modifications because they allow Xtal to play nicely with Louis Farrugia's WinGX suite in a MS Windows environment. In addition the program MAPXCH has been added which aims to read in Wien2k electron density maps and ABINIT valence density maps (both subject to limitations), for contouring and plotting in Xtal.

STARTX has now been extensively rewritten. All elemental data tables previously embedded within the source are now externalized in a CIF format elemental database. That database also contains the multiwavelength atomic dispersion and absorption correction factors calculated and compiled in S.Sasaki (1989), KEK Report, 88-14, 1-136 and S.Sasaki (1990), KEK Report, 90-16, 1-143. Consequently STARTX automatically retrieves these factors for wavelengths in the range 0.1 to 2.8 Angstroms. The CIF elemental table is however rather large and reading data from the CIF is now a significant fraction of the total computation time of typical multiprogram calculations. With a 1Ghz cpu This could take up to 10 seconds. In STARTX upd mode, reading of the atomtype.cif is eliminated where practical.

Xtal is becoming increasingly dependent on external libraries and software. In addition to tcl/tk, this version gratefully acknowledges the use of CIFtbx Version 2.6.3 (Hall, S. R. and Bernstein, H. J., J. Appl. Cryst. (1996). 29, C 598-603). Furthermore, with the introduction of the new program SURFIN, Xtal now incorporates the Togl OpenGL module for Tcl/Tk.

The adoption of Tcl/Tk based graphics in Xtal3.7 certainly aided with the portability of the graphically oriented programs. There were however some shortcomings with the tcl/tk library version dependencies which hopefully have been resolved now in a more acceptible manner. Now by statically linking against a stubs library, the actual runtime library used can be specified by environment variables, rather than a specific library linked against at compilation time. Previously this wasn't such an important issue, but new improved tcl/tk libraries are continually being produced and these will hopefully all be compatible with the current version of Xtal.

A new program EXPAND has been added to aid in conversions of structures between different symmetries, and possibly different cells. It permits the expansion of the cell contents up to three or more unit cells, and then the contraction back to some unique set once the cell setting or symmetry have been changed.

Program documentation in the form of this manual is now encoded using the DocBook SGML dtd and rendered into online html form using the DocBook dsssl stylesheets and OpenJade. This also provides a route to hardcopy printable versions of the manual via pdfjadetex. A HTML based document detailing how to program in RATMAC for Xtal is available separately, along with a HTML based manual for the now no longer supported XTAL programs.

The Xtal binary executable originally read only from the stdin filestream and wrote only to the stdout filestream. From version 3.7 arguments on the command line passed to the binary were assumed to be input filename[.dat|.inp] and output filename[.lst] filenames. Two command line options were then added; -n which supresses treatment of the first argument as an input filename, and -p which forces output listings to be printed to standard output filestream. In the event that only one input filename argument is provided and in the absence of the -p optional argument, an output filename[.lst] is interpolated from the supplied input filename. This facilitates the use of xtal within a MS Windows environment (in particular WinGX v≥1.64.05), where filenames with the extensions [.dat] can be configured to activate the xtal binary directly (via double mouse clicks). For use in this manner it is necessary to have the following environment variables set globally within the users working environment:

Xtal behaviour under UNIX and Microsoft Windows environments should be largely identical. Unfortunately VMS support appears unlikely in the forseeable future.