четверг, 21 октября 2010 г.

GSAS vs Fullprof - what I like, what do you like?

I am far from being professional with modern crystallography tools. However, here comes my impression of what I like in these two software packages (GSAS, Fullprof). I do not include comments on JANA2006, another marvelous piece of software. My comments are subjective and there are more comments than tips and tricks to it.

Fullprof. I know, the Fullprof .pcr files are difficult to handle, but I find them more and more attractive with highlighting (my previous post).

  • LeBail fitting is fast. First, you set DUM parameter to 1, Pcr=1, AUT=0 (skip auto assignment to the fitting variables), Number of refined parameters=0 (according to manual). Then, please set JBT=2 and IRF=0 for all phases under consideration. After that you run the .pcr file with the Fullprof, fit background (polynomial or by manual selection of the background points). Setting Pcr=1 is really crucial if you want to prevent messing up your fitting.
  • I really like combination of highlighting of .pcr file syntax with manual numerical assignment of codewords for different fitted parameters. I usually set AUT=0 in the beginning, then fit the background(assigning CODEWORDS 11-61), then I fit the lattice + zero shifts (some other CODEWORDS), then the peak shapes and etc. If now I assign the Number of fitted parameters only to 6, I will fit only the background. Then, I can slowly increase the Number of fitted parameters and fit the lattice and so on. Only if I am sure I will not mess up the fitting, I can set the AUT=1. I find this way of selecting the parameters to fit is faster, specially with LeBail fitting, than that checkbox style of GSAS.
  • Setting up the magnetic structure is way faster in Fullprof compared with the GSAS.
  • I like the way how simulations work in Fullprof. However, there is an important trick to it. It looks like Fullprof does not like really large numbers (version 2000/2006 in peak intensities/heights). If you have a really strange simulation diffraction pattern with peaks cut off to 0 intensity in the middle, pay attention to: scale of the phase, ATZ of the phase (take a look in .out file), occupation numbers. These are the pictures, first shows bad simulation, second - good.

  • RTFM:)
  • Yes, GSAS has a way much better tutorials. The combination of CMPR+EXPGUI is nicely explained in the same tutorials.
  • The simulation of the diffraction spectra (except the magnetic phase) works like a breeze in GSAS.
  • RTFM is really important.
  • The texture fitting of the GSAS is more sophisticated than in Fullprof. If I have strange textures in my material, I will prefer GSAS.
  • Background fitting in GSAS is outstanding, different models, fitting of the manually selected background points.

Of course, there are many things I do not like, but they are minor. Usually, I have many complains about the user interface. But this is not the complain I would express to the authors of these packages. The guys have done a brilliant job. Thank you guys!!!
However, I think, that investing whole lot of money and research to the creation of even better tools with the cooperation with the IT guys should be at the same priority as investing in the International Tables of Crystallography, International Crystallography Database and their support. I would like to have better tools.

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