Date: 22 Sep 99 18:00:09 WET DDST From: Andrew WillsTo: RIETVELD_L Distribution List Subject: Re: [Theoretical Neutron Patterns] Reply-To: RIETVELD_L Distribution List >I also need to calculate theoretical ND powder patterns, using ToF >data. I am an avid user of GSAS, but I am unsure how to use it to >just calculate theoretical patterns, without reading in a histogram. >I would also like to know if there is a way to obtain a list of the >calculated reflections; d-spacings, hkl, intensities...., as I need >to compare various models, wrt the intensities. >So far the best option I have found for calculating theoretical ND >patterns is to use "Crystallographica", but I am still at the early >stages of trying it out. >Caroline Dear Caroline, forgive me I am on a computer without GSAS, but if memory serves: In GSAS when you want to simulate a diffraction pattern you go to the insert histogram menu (when you begin EXPEDT type: y p h ) and enter 'j'. This inserts a dummy histogram, i.e. one that does not exist. GSAS still needs to work with a histogram file, even if it only holds the limits for the calculations. (You will require an instrument parameter file as usual.) Edit this histogram and define your time of flight limits. If you have already entered all the structure/ profile parameters you can then run POWPREF and after that GENLES. The GENLES cycles will calculate the theoretical pattern. GSAS gets a bit confused with the histogram scale factor so you will probably need to change this to a bigger number by hand. When you are ready to refine real data, you should either tell GSAS not to use the dummy histogram or to replace it for the one that contains the data. The process is the same for reactor data as it is the instrument parameter file that tells GSAS about the type of data. There are also no extra difficulties associated with calculating theoretical patterns for magnetic structures. -Andrew ------------------------------------------- Andrew S. Wills (Dr.) SPSMS/DRFMC/MDN Centre D'Etudes Nucléaires de Grenoble 17 Avenue des Martyrs Cedex 9 Grenoble, 38054 France fax: (33) [0]4 76 88 51 09 telephone: (33) [0]4 76 88 54 63 email: wills@drfmc.ceng.cea.fr