XABS2: an empirical absorption correction program

The XABS2 Fortran program calculates an empirical absorption correction based on minimization of the 2 differences between F²_obs and F²_calc. 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 relationship for spherical crystals between the transmission factor and sin² θ for θ ≃ 30° when μR ≃ 5. In XABS2, deviations from linearity are accounted for by a cubic equation in sin² θ, which makes it applicable over the whole range of diffraction angle. The program needs no data in addition to the unique set but assumes a bisecting (symmetric) data-collection mode on a four-circle diffractometer. Since it does not require knowledge of the diffractometer setting angles, it can be applied even in cases where details of the original data collection are unknown.