Updates in SASfit for fitting analytical expressions and numerical models to small-angle scattering patterns.

Small-angle scattering is an increasingly common method for characterizing particle ensembles in a wide variety of sample types and for diverse areas of application. SASfit has been one of the most comprehensive and flexible curve-fitting programs for decades, with many specialized tools for various fields. Here, a selection of enhancements and additions to the SASfit program are presented that may be of great benefit to interested and advanced users alike: ( a ) further development of the technical basis of the program, such as new numerical algorithms currently in use, a continuous integration practice for automated building and packaging of the software, and upgrades on the plug-in system for easier adoption by third-party developers; ( b ) a selection of new form factors for anisotropic scattering patterns and updates to existing form factors to account for multiple scattering effects; ( c ) a new type of a very flexible distribution called metalog [Keelin (2016). Decis. Anal. 13 , 243-277], and regularization techniques such as the expectation-maximization method [Dempster et al. (1977). J. R. Stat. Soc. Ser. B (Methodological) , 39 , 1-22; Richardson (1972) J. Opt. Soc. Am. 62 , 55; Lucy (1974). Astron. J. 79 , 745; Lucy (1994). Astron. Astrophys. 289 , 983-994], which is compared with fits of analytical size distributions via the non-linear least-squares method; and ( d ) new structure factors, especially for ordered nano- and meso-scaled material systems, as well as the Ornstein-Zernike solver for numerical determination of particle interactions and the resulting structure factor when no analytical solution is available, with the aim of incorporating its effects into the small-angle scattering intensity model used for fitting with SASfit .