Dear Bhuv,
yes it is possible to evaluate it. Normally it should be sufficient to compute the F2 index from the spherical harmonic coefficient.
The problem may be is that TOPAS does not have such computation inside. So the other way is to export the coefficients, to hope they are some standard one and input them in a texture program that can compute F2 or texture strength index.
There could be some problems:
- if you have reduced the image to only one spectrum than using the harmonic apparatus what you get as F2 it could be it is not at all representative of the real texture strength as you are not fitting the real texture.
- as the texture coverage you have is for sure very small and insufficient, the harmonic coefficient will be strongly correlated with other errors and as the harmonic is not robust at all in such cases you are likely to get big errors in F2.
I would suggest one thing. If the Rwp decreases only 1%, why to use the harmonic texture correction at all?
A texture correction should be used (and especially the harmonic one) having a high level of confidence on the method, its effect and knowledge of the real texture of your sample and geometry/texture connections. Otherwise is just a black box that may lead to more errors than corrections.
Best regards,
Luca Lutterotti
On Apr 1, 2005, at 17:44, P. Bhuv wrote:
Hi,
I would like to discuss a couple of questions raised by a referee;-( I am not sure if it is ethical or not - I thought it is better to ask the way than .....
1.
I have used very (very very)small amount of sample to collect a powder pattern and I have corrected possible graininess with spherical harmonics to account for small discrepancies in some of the peaks (to give an idea - the Rwp decreased only 1% on correction - if the numbers mean anything). Is it possible to estimate the level of preferred orientation, if one uses a spherical harmonics correction? (I know preferred orientation is not the right term to use here, in the first place)
2.
The data I have collected is on a image plate (only one frame). Also, I have used a z-matrix (rigid body) and allowing small meaningful changes in bond lengths, angles, and torsions, i.e., z-matrix of the type (in TOPAS):
A1 X2 A1 1.855 min 1.8 max 1.9 X3 X2 1.649 min 1.6 max 1.7 A1 114.987 min 113 max 116 X4 X3 1.649 min 1.6 max 1.7 X2 x4x3x2 122.80 min 122 max 123 A1 169.5 min 160 max 175 ........................ ........................ ........................ ........................
In such a case, is it possible to give (any meaningful) Standard Uncertainties for the atom positions, (also) given the fact that the atoms are not refined independently?
Thanks for any wise comments in advance,
Bhuv
------------------------ N. Bhuvanesh Texas A&M Univ. Coll. Stn., TX ------------------------
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