Section 25.2.10.2. Program organization and philosophy

SHELX is written in a simple subset of Fortran77 that has proved to be extremely portable. The programs SHELXS (structure solution) and SHELXL (refinement) both require only two input files: a reflection file (name.hkl) and a file (name.ins) that contains crystal data, atoms (if any) and instructions in the form of keywords followed by free-format numbers etc. These programs write a file, name.res, that can be renamed or edited to name.ins for the next refinement and can output details of the calculations to name.lst. Although originally designed for punched cards, this arrangement is still quite convenient and has retained upwards compatibility for the last 30 years. The common first part of the filename is read from the command line by typing, e.g., `SHELXL name'. The programs are executed independently without the use of any hidden files, environment variables etc.

The programs are general for all space groups in conventional settings or otherwise and make extensive use of default settings to keep user input and confusion to a minimum. Particular care has been taken to test the programs thoroughly on as many computer systems and crystallographic problems as possible before they were released, a process that often required several years!