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Methods, Problems and Solutions

GSAS (General Structure Analysis System) Rietveld powder diffraction and Single Crystal software

GSAS - How to calculate patterns in GSAS - including Theoretical Neutron Patterns

The CCP14 Homepage is at http://www.ccp14.ac.uk

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[The reference to use for GSAS in any resulting publications is: A.C. Larson and R.B. Von Dreele, "General Structure Analysis System (GSAS)", Los Alamos National Laboratory Report LAUR 86-748 (1994).]

How to apply Occupancy constraints with more than two variables in GSAS.

Via Rietveld Mailing List

Date: 22 Sep 99 18:00:09 WET DDST
From: Andrew Wills 
To: 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.


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


Andrew S. Wills (Dr.)
Centre D'Etudes Nucléaires de Grenoble
17 Avenue des Martyrs
Cedex 9
Grenoble, 38054

fax:       (33) [0]4 76 88 51 09
telephone: (33) [0]4 76 88 54 63 
email:   wills@drfmc.ceng.cea.fr

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