[Ifeffit] Amplitude and sigma square

[Scott Calvin]

Hi all,

> On May 29, 2018, at 11:35 AM, Carlo Segre <segre at iit.edu> wrote:
> 
> Finally, the answer to your second question is that you do NOT need to fit longer paths if all you want to know is the first shell information.

As a rule of thumb, of course, this is true. But there are two situations I think are worth noting, even though I think neither applies to Abdul’s particular example.

One is with a well-characterized system for which highly accurate first-shell information is desired; e.g. bond lengths within a hundredth of an angstrom or so. Outer shells can contribute some in the Fourier transform in the range being used to fit the first shell. The effect is likely to be modest, and for some purposes can be considered negligible, but when very high accuracy is desired it may be relevant.

The other is when the long-range structure is roughly known, but distortions relative to the model structure are expected. For example, substitutional doping might leave a crystal with the same overall structure, but with both long-range differences (e.g. Vegard’s law type stuff) and local distortions. In these cases, including paths beyond the first shell constrained in a physically defensible manner can help reduce the correlations between fitted parameters and lead to smaller uncertainties in the fitted parameters. For example, the mixed-metal ferrite systems I’ve worked with were like this.

Best,

Scott Calvin
Lehman College of the City University of New York

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