[sdpd] background

[ouyang...@mail.chem.tamu.edu]

cockcroft...@gordon.cryst.bbk.ac.uk wrote:

> In reply to Jaap's last comment,
>   You can subtract the background in PRACTICE too after refinement as
> long as you know the function used for the refinable background (e.g.
> the polynomial coefficients when a polynomial is used.) This can
> be done each cycle of refinement (and is trivial to code). Whether
> the peaks overlap or not is irrelevant - or even whether you can see
> the background between peaks.
>   After each cycle of refinement, simply do
>      yi(calc. diffraction)' = yi(calc. total) - yi(calc. background)
> and  yi(obs. diffraction)' = yi(obs. total) - yi(obs. background)
> and then use yi' values for calculating Rwp, Rexp, etc. At least Rwp then
> bears a better relationship to the structural side of the refinement.
>   Whether the commonly used codes do this or not is another matter. But
> just because some software doesn't calculate the "correct" numbers isn't
> a justification (IMHO) for publishing "wrong" ones. (Obviously the
> words in quotes are an my opinion only.)
>   Jeremy

Some pratical questions.
How many fix background points/ refinable background terms are reasonable?
GSAS can let you use up to 99 fix background points and 36 refinable background
terms.
The examples in the tutorial use about 4-6  (There is no polynomial type
function in GSAS.
I have been suggested to use cosine fourier series for in house data.)
Are there articles comparing different background functions?
Thanks.

Sean X. Ouyang
Department of Chemistry
Texas A&M University