One of the reasons could be preferred orientation. If this is the case, one could derive orientation axis from the ellipsoids.
Peter Zavalij X-ray Crystallographic Center University of Maryland College Park, MD Office: (301)405-1861 Lab: (301)405-3230 Fax: (301)314-9121 -----Original Message----- From: Lee Gerrard [mailto:lee.gerr...@awe.co.uk] Sent: Monday, August 03, 2009 11:35 AM To: rietveld_l@ill.fr Subject: Negative Equivalent Isotrpic Calculated Values Problem.. 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 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ This e-mail and any attachments may contain confidential and privileged information. If you are not the intended recipient, please notify the sender immediately by return e-mail, delete this e-mail and destroy any copies. Any dissemination or use of this information by a person other than the intended recipient is unauthorized and may be illegal. No virus found in this incoming message. Checked by AVG - www.avg.com Version: 8.5.392 / Virus Database: 270.13.43/2280 - Release Date: 08/03/09 17:56:00
