support sratio and rcut in cmi

[dragonyanglong]

In diffpy-cmi, there is no sratio and rcut functions available.

Sigma_ij = sqrt( sigma_isigma_j + f(delta1) + f(delta2) )
Sigma_ij is the width of the peak jointing atom i and atom j. Sigma_i is the adp of atom i.
If r_ij < rcut:
sigma_ij -> sigma_ijsratio
Ex: if rcut = 4.0, peak 3.95 A then we expand it via sratio so it gets to 4.1A.

[dragonyanglong]

Hi @sbillinge , I dig into our previous email threads, and I found this issue. I think CMI does not support sratio and rcut. You and I discussed some temporary solution to wrap it at the pdfgui end, instead of touching the cmi source codes. wrote down some temporary wrap up function.

[sbillinge]

Hi @sbillinge , I dig into our previous email threads, and I found this issue. I think CMI does not support sratio and rcut. You and I discussed some temporary solution to wrap it at the pdfgui end, instead of touching the cmi source codes. wrote down some temporary wrap up function.

Thanks @dragonyanglong . Let's look at this this summer. I will have some people helping but they will need some mentoring.

[sbillinge]

A general thought from @sbillinge:

the rcut/sratio is kind of brutal. I wonder if we can kind of bootstrap it by changing the rcut into something like a Fermi function that crosses over rapidly at some point in r and simply multiply this by sratio. We could have it go over to 1 at low r and zero at high-r and then maybe add this to our sigma model as an additional term. Could something like that work? Anyway, something along those lines.