Re: Size Strain in GSAS

[Nicolae Popa]

Title: Message

Alan, But the analytical representation of the profile, even by empirical functions, also helps in the analysis of Size/Strain you don't think? You don't agree with 3 Lorentzians even if they are sharper than two pVs? Probably is a reason that I don't see.   Concerning the numerical derivatives probably the providers of popular codes have an opinion. Personally I use quite exclusively the numerical gradients in my personal least square program but I know that many people avoid, if possible.   Best wishes, Nicolae   Nicolae >To resume, I think (for example) that is better to approximate by a sum of >three Lorentzians (involving 4 profile parameters) than by a sum of two >pVs (involving 5 profile parameters).   I couldnt agree more.   >Concerning the numerical calculation of the profiles, still I'm not convinced >that is the ideal solution. You have not only the size profile to calculate, >but also at least two convolutions, with the strain and the instrumental >profiles. Moreover, what are doing the codes that use the gradient >calculated analytically?   Size distribution is a start and Leonie does seem to have worked in some of the other effects to his credit.   Whether all this actually helps in the analysis of Size/Strain does not seem to concern many - so why not experiment.   By gradient, I presume you mean the derivatives of the distribution with respect to parameters. This would require a mixture of numerical and analytical derivatives - very simpe using the chain rule. Dont see why the same cant be done in other codes.   all the best alan