RE: R-Value SHELX/GSAS
Dear Friedrich, There are inequivalent reflections that overlap even in cubic structures; those where h^2+k^2+l^2 are the same will have identical d-spacings (e.g. 300 & 221, 410 & 322). Check through the list of reflections to see those that have identical d-spacings; these are also listed in the Intl. Tables for X-ray Crystallography Vol II (red book) p 125ff. A LeBail refinement will partition the intensity in some way (probably equally) that is not likely to be what the real ratio is. 2nd point is name given by SHELX to that R-value, wR2, implies some use of a weight. The R(F**2) in GSAS is not weighted. Why this all results in a factor of six; I guess I'm a bit surprised. Best, Bob Von Dreele R.B. Von Dreele IPNS Division Argonne National Laboratory Argonne, IL 60439-4814 -Original Message- From: Friedrich W. Karau [mailto:[EMAIL PROTECTED] Sent: Tuesday, October 17, 2006 9:15 AM To: rietveld_l@ill.fr Subject: R-Value SHELX/GSAS Dear colleagues, may be, that I have comprehension problem. The Bragg R-value is named R(F**2) in GSAS and wR2 in SHELX and defined in the same manner. when I extract the reflexion intesities using a LeBail fit and write them into a hklf6 formatted file, SHELX should refine the structure resulting similar Bragg R-values like GSAS. The fact is, the the R-values obtained by SHELX are about 6 times bigger than the R-values obtained by GSAS. The structure is cubic I, so that no overlap of the reflections occurs apart from the multiplicity, that is incorporated in the hklf6 file. However the structure refines and converges very well in both cases. I have testet refinement with both SHELX and GSAS for a hexagonal structure, too, but the result is the same. The R-values obtained by SHELX are about 6 times bigger than the R-values obtained by GSAS. What could be the reason? Thank you in advance Friedrich + Friedrich W. Karau email: [EMAIL PROTECTED] + + AK-Schnick Tel.: (+49) (0)89 - 2180-77449 + + LMU - Muenchen mobil: 0179 6647667 + + Department Chemie u. Biochemie Fax: (+49) (0)89 - 2180-77440 + + Butenandtstrasse 5-13(D) URL: www.cup.uni-muenchen.de/ac/schnick + + D-81377 Muenchen +
Re: R-Value SHELX/GSAS
Dear Friedrich, Normally, the Bragg R-factor is underestimated as it is calculated via a single application of the Rietveld decomposition formula which underestimates the difference between Icalc and Iobs for partly overlapped peaks. When the Rietveld decomposition formula is applied several times, as it is done in the Le Bail procedure, then the values of Iobs become closer to the real observed intensities and the difference between Icalc and Iobs may become much higher depending on the peak overlap. Best regards, Leonid Solovyov -Original Message- From: Friedrich W. Karau [mailto:[EMAIL PROTECTED] Sent: Tuesday, October 17, 2006 9:15 AM To: rietveld_l@ill.fr Subject: R-Value SHELX/GSAS Dear colleagues, may be, that I have comprehension problem. The Bragg R-value is named R(F**2) in GSAS and wR2 in SHELX and defined in the same manner. when I extract the reflexion intesities using a LeBail fit and write them into a hklf6 formatted file, SHELX should refine the structure resulting similar Bragg R-values like GSAS. The fact is, the the R-values obtained by SHELX are about 6 times bigger than the R-values obtained by GSAS. The structure is cubic I, so that no overlap of the reflections occurs apart from the multiplicity, that is incorporated in the hklf6 file. However the structure refines and converges very well in both cases. I have testet refinement with both SHELX and GSAS for a hexagonal structure, too, but the result is the same. The R-values obtained by SHELX are about 6 times bigger than the R-values obtained by GSAS. What could be the reason? Thank you in advance Friedrich + Friedrich W. Karau email: [EMAIL PROTECTED] + + AK-Schnick Tel.: (+49) (0)89 - 2180-77449 + + LMU - Muenchen mobil: 0179 6647667 + + Department Chemie u. Biochemie Fax: (+49) (0)89 - 2180-77440 + + Butenandtstrasse 5-13(D) URL: www.cup.uni-muenchen.de/ac/schnick + + D-81377 Muenchen + __ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com
Re: R-Value SHELX/GSAS
Just had a look at an old refinement. Shelx came out with a much lower wR2 (few percent) than GSAS (12/14 %). I think I used hklf 5 and shelx should sum over peaks, not the contributing reflections as in Rietveld. Would have to investigate further, but I notice shelx had a gof (or chi^2) of 1 and fiddles with the weights to make it so. Given the fit in question was pretty good (for LaB6) it does seem surprising that R(F**2) from Rietveld is suggesting that the intensities are on average wrong by more than 10%. I would also have expected the Rietveld value to be too low, but it came out the other way around. Maybe due to summing over more peaks. How about your case Freidrich? Lots of people must have a silicon pattern around for testing this out themselves... Best, Jon On Tue, 17 Oct 2006, Leonid Solovyov wrote: Dear Friedrich, Normally, the Bragg R-factor is underestimated as it is calculated via a single application of the Rietveld decomposition formula which underestimates the difference between Icalc and Iobs for partly overlapped peaks. When the Rietveld decomposition formula is applied several times, as it is done in the Le Bail procedure, then the values of Iobs become closer to the real observed intensities and the difference between Icalc and Iobs may become much higher depending on the peak overlap. Best regards, Leonid Solovyov -Original Message- From: Friedrich W. Karau [mailto:[EMAIL PROTECTED] Sent: Tuesday, October 17, 2006 9:15 AM To: rietveld_l@ill.fr Subject: R-Value SHELX/GSAS Dear colleagues, may be, that I have comprehension problem. The Bragg R-value is named R(F**2) in GSAS and wR2 in SHELX and defined in the same manner. when I extract the reflexion intesities using a LeBail fit and write them into a hklf6 formatted file, SHELX should refine the structure resulting similar Bragg R-values like GSAS. The fact is, the the R-values obtained by SHELX are about 6 times bigger than the R-values obtained by GSAS. The structure is cubic I, so that no overlap of the reflections occurs apart from the multiplicity, that is incorporated in the hklf6 file. However the structure refines and converges very well in both cases. I have testet refinement with both SHELX and GSAS for a hexagonal structure, too, but the result is the same. The R-values obtained by SHELX are about 6 times bigger than the R-values obtained by GSAS. What could be the reason? Thank you in advance Friedrich + Friedrich W. Karau email: [EMAIL PROTECTED] + + AK-Schnick Tel.: (+49) (0)89 - 2180-77449 + + LMU - Muenchen mobil: 0179 6647667 + + Department Chemie u. Biochemie Fax: (+49) (0)89 - 2180-77440 + + Butenandtstrasse 5-13(D) URL: www.cup.uni-muenchen.de/ac/schnick + + D-81377 Muenchen + __ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com
