The Gnu Xtal System is a reasonably comprehensive, modular suite of software oriented primarily towards small molecule crystallography (with a smattering of powder and charge density work). The software documented herein is directly descended from the Xtal System (with only a minor name change) and thereby from the original XRAY76 suite. It was conceived, created and maintained for many years by Syd Hall (the originator of the Crystallographic Information File (CIF) at the University of Western Australia. Numerous other developers have contributed to this suite over the years.
One of the most novel features of Xtal is its use of the RATionalized MACro (RATMAC) preprocessor to insulate the code base from variations in F77 dialects and operating system specific functionalities which were prevalent when the system was first developed. Use of the RATMAC preprocessor also encouraged a coding style in which almost every single line of code was documented, aiding both the development and maintenance of the code base. The modular design of the software and its use of archives for conveying calculation results between different program modules accessed by a nucleus of common IO routines has resulted in a very robust and stable program suite and facilitated the development of diverse calculation modules by many different authors. In fact an earlier release of the the system contained more than 70 separate calculation modules.
The Gnu Xtal System is then a reasonably extensive and powerful set of portable crystallographic routines, with incredibly well documented source code and more than 200 pages of usage notes, eminently suited to the addition of new modules by new developers (hint #1). It does however have two serious limitations. Firstly, it is written in a language many people now find archaic, that being fortran77, albeit tastefully wrapped in RATMAC and secondly, it's size imparts a significant degree of inertia, such that it cannot readily be modified to new programming styles and languages (although the use of RATMAC might aid in such a transition if sufficiently motivated people were dedicated to such a goal, hint #2).
Consequently the past and present authors, developers and maintainers of the the Xtal System present here, for your crystallographic pleasure, the Gnu Xtal System, available as an open source project, with the sincere belief that it is useful, in the hope that community support may add renewed vigour to the behemoth, but with the expectation that Sourceforge may be its twilight resting place, afore it's entombed by the sands of time, right alongside The King of Kings.
The Gnu Xtal System is distributed under the GNU General Public License.