<i>XABS</i>2: an empirical absorption correction program

TLDR

The XABS algorithm has been employed for over a decade at the University of California, Davis, originally assuming a linear relationship between transmission factor and sin²θ for spherical crystals at moderate angles. XABS2, a Fortran program, computes an empirical absorption correction by minimizing the difference between observed and calculated F², using a cubic equation in sin²θ to capture nonlinearity and requiring no additional data beyond the unique set while assuming a bisecting symmetric data‑collection mode on a four‑circle diffractometer. Because it does not depend on diffractometer setting angles, XABS2 can be applied even when the original data‑collection details are unknown.

Abstract

The XABS2 Fortran program calculates an empirical absorption correction based on minimization of the 2 differences between F2obs and F2calc. 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.