[GSAS-II] Gaussian FWHM - sig

[Marcus Miranda]

Hello,

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.

Sincerely,

Marcus A. R. Miranda
Universidade Federal do Ceará - UFC
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