As far as I understand, the major issue is that BG subtraction changes the error statistics (accordingly, weights applied in calculating function to minimize...). It is also somewhat arbitrary how you choose the BG line - so, you can easily subtract part of Bragg contribution from base of peak - hence, bias the data. Most probably there are more things I didn't mention. So, of course it is preferable to fit BG with some suitable empirical function (e.g. Chebyschev polynomials).
However, some Rietveld programs do not have good selection of BG functions, and it is sometimes problematic to correctly fit the BG - so, I think in this case careful BG removal would be the only option. I'd like to use this opportunity to ask Fullprof experts: is the Chebyschev option finally implemented? Or otherwise what is the best option to model difficult (wavy) background with Fullprof? Sincerely, Maxim. ------------------------------------------- Dr. Maxim Lobanov R&D Director Huntsman-NMG mailto: m_loba...@huntsman-nmg.com ********************************* If you encounter any difficulties sending e-mails to the addresses in huntsman-nmg.com domain, this could be due to the our spam filter malfunction. In case of such an event please send a message to n...@fromru.com Please note that the old domain nmg.com.ru does not exist anymore - please update your address book accordingly -----Original Message----- From: matthew.row...@csiro.au [mailto:matthew.row...@csiro.au] Sent: Wednesday, March 04, 2009 8:32 AM To: olga.smirn...@hw7.ecs.kyoto-u.ac.jp Cc: rietveld_l@ill.fr Subject: RE: cRs It's German... "verboten" == forbidden. You should never delete the background from a diffraction pattern prior to (Rietveld) analysis. It changes peak positions and shapes and generally is bad. Cheers Matthew ________________ Matthew Rowles CSIRO Minerals Box 312 Clayton South, Victoria AUSTRALIA 3169 Ph: +61 3 9545 8892 Fax: +61 3 9562 8919 (site) Email: matthew.row...@csiro.au -----Original Message----- From: Olga Smirnova [mailto:olga.smirn...@hw7.ecs.kyoto-u.ac.jp] Sent: Wednesday, 4 March 2009 16:30 To: Brian O'Connor Cc: rietveld_l@ill.fr Subject: Re: cRs According to my computer, I received your response 4 minutes earlier than forwarded the email. What is 'verbotten' (volabulary does not find) ? OS Brian O'Connor wrote: > Background removal is verbotten! > Brian O'Connor > > ________________________________ > > From: Olga Smirnova [mailto:olga.smirn...@hw7.ecs.kyoto-u.ac.jp] > Sent: Wed 4/03/2009 2:14 PM > To: rietveld_l@ill.fr > Subject: cRs > > > > Dear All, > > How is life with conventional R factors when you always have to divide > by zero background? > Let's have time. Considering a part of the profile without peaks one > gets 100% cR. > I did not give the agreement factor; I would say those cR with all > non-excluded points is incorrect, but cR for > points with Bragg contribution is almost the same! > Do you decrease the Rs by adding the background or do you increase cRs > by subtracting the background? > > OS > > PS I did not ask my supervisor before sending such a mail. > > > > > > ------------------------------------------------------------------------ > > Internal Virus Database is out-of-date. > Checked by AVG. > Version: 7.5.519 / Virus Database: 269.21.6/1323 - Release Date: 3/10/2008 > 11:07 AM >
