[Discus-talk] [Fwd: Setting variables in DISCUS PDF module]

[<rei...@fa...>]

Hi Kris,

Let me answer your questions:

*Neutron scattering wavelength:*
  IN DISCUS I set the neutron scattering length in the Fourier menu. The
  PDF menu does not offer a modification.
read
   cell nickel.cell
#
fourier
   neutron      ! switch to neutron scattering
exit
show scat, all  ! Show current scattering length
four
   scat ni, 0,0, 0,0, 0,0, 0,0,  10.05  ! modify
                 ! even though its a neutron scattering length
                 ! one needs to set a(i), b(i), c as for X-ray...
exit
show scat, all   ! Show modified scattering length

*Scaling experimental data:*

See the attached demo macro, which creates a Nickel PDF and
then modifies the Nickel structure, scale factor and refines
to match the "experimental data"
I realised that the on-line help is missing the info on the
"set refine" command within the PDF menu... To be corrected.

Best

Reinhard Neder


----------------------------------- Original Message -----------------------------------
Subject: [Discus-talk] Setting variables in DISCUS PDF module
From:    "Kris Harris" <kri...@mc...>
Date:    Thu, May 26, 2016 22:10
To:      dis...@li...
----------------------------------------------------------------------------------------

Hi,

As a first exercise, I'm attempting to reproduce an RMC fit of a neutron
PDF from a published article.  I've gotten close but don't know how to
change two parameters:

*Neutron scattering wavelength:* How do I read out (and change if needed)
these values?  The command "show" produces a section called "Weights for
pdf calculations", but I don't see how to connect this to values of b_i in
the equation given in the manual for G(r).

*Scaling experimental data:*  My initial simulations look close, but I need
to scale the entire simulation smaller to about 80%  to match the
experimental data. I'm guessing it's theoretically OK to do this? The
manual lists a variable pdf_scal[i] but I can't seem to figure out the
syntax to set it. I guess scaling the value of the average scattering
power, <b>, would do the same thing, but I'm not sure how to do that either.

thanks for any help,
Kris
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-- 
Prof. Dr. Reinhard Neder
Kristallographie und Strukturphysik
Universität Erlangen
Staudtststr. 3; 91058 Erlangen
tel. +49-9131-8525191
fax  +49-9131-8525182