• small-angle scattering;
  • data analysis;
  • computer programs;
  • ab initio methods;
  • rigid-body modelling.

The program package ATSAS 2.1 for small-angle X-ray and neutron scattering data analysis is presented. The programs included in the package cover the major processing and interpretation steps from primary data reduction to three-dimensional modelling. This system is primarily oriented towards the analysis of biological macromolecules, but could also be used for non-biological isotropic and partially ordered objects (nanoparticle systems, colloidal solutions, polymers in solution and bulk). Recent developments in the programs included in ATSAS 2.1 are highlighted. The main programs run on multiple hardware platforms, including Windows PC, Linux RedHat and Suse, DEC Alpha, SGI IRIX and Mac OSX.