ABS: a program to determine absolute configuration and evaluate anomalous scatterer substructure

In single- or multi-wavelength anomalous dispersion (SAD/MAD) phasing, one of the essential steps is to find the absolute configuration (hand) of the anomalous scatterers. A computer program, ABS, based on an algorithm proposed by Woolfson & Yao [Acta Cryst. (1994), D50, 7-10] has been written to determine the absolute configuration by using anomalous scattering data. It also calculates a real-space figure of merit (FOM) that can be used to assess the quality of the solution for anomalous scatterer sites. The ABS program has been successfully applied in several ab initio phasing cases, including some previously unknown protein structures. The program is included in the CCP4 suite.