All,

I am refining some TOF powder neutron diffraction data (GEM Xpress)
using GSAS, when I refine with isotropic displacement parameters I get
reasonable and consistent values. However, when I refine using
anisotropic displacement parameters the equivalent isotropic calculated
from these are almost all negative. Furthermore, there is a large
improvement in the goodness-of-fit parameters when using the anisotropic
displacement parameters. The improvement in the fit appears to be a
generally better fit to the intensities over all the data rather than
any identifiable area.

Does anyone have any idea why this should be occurring and maybe offer
some assistance please? 

Regards,

Lee

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dr. Lee A. Gerrard CSci CChem MRSC
Materials Scientist 

Materials Science Research Division
SB40.1
AWE Plc
Aldermaston, Reading,
Berkshire
RG7 4PR 

Tel:  +44 (0)118 982 6516
Fax:  +44 (0)118 982 4739
Email: lee.gerr...@awe.co.uk
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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