Yell: a computer program for diffuse scattering analysis via three-dimensional delta pair distribution function refinement

Yell, a program for routine refinement of disorder models against single-crystal diffuse scattering data, is presented. The analysis is based on the three-dimensional delta pair distribution function (3D-ΔPDF) method, which provides direct access to interatomic correlations in real crystal structures. Substitutional, displacive and size-effect disorder models are covered. The input file format supports flexible usage of arithmetic expressions for constraining dependent parameter values. The program is designed to be run on desktop computers. By using an efficient fast-Fourier-transform-based diffuse scattering calculation algorithm, full least-square refinements of medium complexity disorder models may be performed within minutes or hours, even if the experimental diffuse scattering is represented by large and fine-sampled reciprocal space volumes. The program is written in C++ and the source code is distributed under the GPL licence. Binary distributions are currently available for Mac and Windows operating systems.