If the diffraction instrument’s optical properties are well characterized, then it is possible to construct a description of the expected peak profiles using the fundamental parameters approach (FPA). While FPA models have been developed for synchrotron and neutron diffractometers, this is primarily the domain of laboratory x-ray diffractometers. GSAS-II provides a module where FPA terms may be input and an automatic process then
generates peak profiles as a function of \(2\theta \) and fits them with the GSAS-II profile coefficients. These profile terms are then used to generate an instrument parameter file. The FPA profiles are generated with the NIST “Fundamental Parameters Python Code” from Marcus H. Mendenhall et al. that is included in GSAS-II. To access this capability, use the “Fit instr. profile from fundamental parms” command in the Powder Diffraction submenu in the Imports menu; also see the “Determining Profile Parameters with Fundamental Parameters” tutorial.