DIFRAC
DIFRAC introduction
Users of several models of 4-circle single-crystal diffractometers may well
have been struck by the diversity of form and content of the data files
generated by diffractometer-manufacturers' software. The form of the file can
create difficulties in its transfer to other computing equipment and further
renders the corresponding computer programs specific to a certain type or
types of diffractometer. The difficulties in the content of the files manifest
themselves principally by the paucity of the available information
necessitating additional input to the data treatment software (e.g. type of
radiation, wavelength of radiation, scan width, sense of rotation of angles on
the single-crystal diffractometer, etc.). The problem has become
aggravated in recent times by the rapid development in electronic data
exchange. It is now frequent to exchange files by computer networking and
electronic mail rather than by the standard means of the recent past, the
1/2in magnetic tape. The advent of machine-readable submission for publication,
data-base and supplementary-material deposition further highlights the problem
of missing data.
Our own motivation for producing this software arose from the work of the
IUCr Subcommittee on Statistical Descriptors (Schwarzenbach, Abrahams
et al., 1989). We wished to be able to perform structure refinements on the
directly-observed quantities such as peak and background intensities
(Schwarzenbach and Flack, 1991), crystal dimensions (Schwarzenbach, 1991;
Blanc, Schwarzenbach and Flack, 1991) etc. which should not be corrected for any
systematic effects. In this project we also had to face the problem of
producing refinement and data-treatment software which would function smoothly
on data produced by three different types of single-crystal diffractometer.
Further it became clear that with the advent of machine-readable submission of
structural papers to journals and data bases it would be highly advantageous to
obtain a clear, complete, instrument-independent record of the diffractometer
measurements and conditions in the form of a data file at the earliest possible
opportunity i.e. while the crystal was still mounted on the instrument.