Introduction to the Program FULLPROF: Refinement of Crystal and Magnetic Structures from Powder and Single Crystal Data Juan Rodríguez-Carvajal Laboratoire Léon Brillouin (CEA-CNRS), CEA/Saclay, 91191 Gif sur Yvette Cedex, FRANCE. In these notes an introduction to the program FullProf is presented. After a brief introduction summarizing the history of the program we present the main elements of the Rietveld method and how to use the program in routine work for refining crystal and magnetic structures. The most specialized topics (microstructure effects, flipping ratio refinements, the use of special form-factors, time of flight neutron powder diffraction) will not be treated here. A full example of Simulated Annealing run, for localizing hydrogen atoms using neutron powder diffraction, is discussed in more detail. These notes have been written taking parts of the manual and other tutorial documents that are available in the FullProf Web site. Introduction The program FullProf has been mainly developed to perform Rietveld analysis [1] of neutron or X-ray powder diffraction data collected at constant, or variable, step in scattering angle 2θ or using the technique of neutron time-of-flight (TOF). Single Crystal refinements can also be performed alone or in combination with powder data. However, the program has some “structure determination” capabilities by using the Simulated Annealing method for global optimization. The first versions of the program FullProf were based on the code of the DBWS program [2], which was also a major modification of the original Rietveld-Hewat program. The program FullProf has been re-written using the full capabilities of the new Fortran 95 standard during 1997-1998. It is progressively being transformed in a program based in the Crystallographic Fortran 95 Modules Library [3]. The program works with some allocatable arrays so the user can directly control the dimensions of important arrays at run time. In this paper we shall describe some elementary points concerning the methods implemented in the program and how to use it. For further details the user should consult the manual and tutorials. The Windows version of the program and all the suite of programs related to FullProf are now distributed within the FullProf Suite installer (setup_FullProf_Suite.exe). This installer, additional documents and tutorials, can be found in the FullProf Web site [4]. The Rietveld Method A powder diffraction pattern can be recorded in numerical form for a discrete set of scattering angles, times of flight or energies. We will refer to this scattering variable as T . Then, the experimental powder diffraction pattern is usually given as two arrays { } 1,..., , i i i n T y = . In the case of data that have been manipulated or normalized in some way the three arrays { } 1,..., , , i i i i n T y σ = , where i σ is the standard deviation of the profile intensity i y , are needed in order to properly weight the residuals in the least squares procedure. The profile can be modeled using the calculated counts ci y at the ith step by summing the contribution from neighboring Bragg reflections plus the background: