I am having difficulties in understanding the "sig" parameter in GSAS 2
under "Reflection List".
I believe GSAS 2 is showing the square of the gaussian fwhm component of
the pseudo Voigt function,
sig = U*tan^2(theta) + V*tan(theta) + W, in centidegrees. In fact, when
using U, V and W from
"Instrument Parameters" I arrive at the value of sig in "Reflection List".
The problem is that the value of "sig" showed by GSAS is quite different
from the value I get when
using another program, for example the XPert HighScore. See example below.
(Note that the "gam" values match)
              sqrt(sig)      gam   fwhm
GSAS      1.7219   6.7518    7.043
Xpert        3.584    6.777      8.37
To calculate the last column, I used : fwhm = Sum(i=0 to 5) [Ki * G^[5-i] *
L^i], in which
G = sqrt(sig) and L = gam. This is the number 3 equation in Thompson, Cox
and Hastings,
J. Appl. Cryst. (1987) 20, 79-83.
Visually the fits from both GSAS and Xpert are pretty good, so I was
wondering if GSAS is
adding/subtracting something else in "sig".
I appreciate your help.


Marcus A. R. Miranda
Universidade Federal do CearĂ¡ - UFC
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