XABS2 - AN EMPIRICAL ABSORPTION CORRECTION PROGRAM

The XABS2 Fortran program calculates an empirical absorption correctio n based on minimization of the differences between F(obs)2 and F(calc) 2. The basic algorithm has been used in the crystallography laboratory at the University of California, Davis for over a decade, in the form of the program XABS. XABS relied upon an approximately linear relatio nship for spherical crystals between the transmission factor and sin2 theta for theta less-than-or-similar-to 30-degrees when muR less-than- or-similar-to 5. In XABS2, deviations from linearity are accounted for by a cubic equation in sin2 theta, which makes it applicable over the whole range of diffraction angle. The program needs no data in additi on to the unique set but assumes a bisecting (symmetric) data-collecti on mode on a four-circle diffractometer. Since it does not require kno wledge of the diffractometer setting angles, it can be applied even in cases where details of the original data collection are unknown.