Average sample strain tensor?

[jonwright]

See if we could fit an average sample strain in the transformation gui. Apply this to g-vectors to make indexing easier. Will need a bit of testing to see if you can limit it to two theta shifts?

[jonwright]

Some progress on this could be. Residual error in guitransformer gives average strain as:

• strain = cot(theta) dtheta

You have directions for each g-vector via (gx,gy,gz)/dstar. You can get a computed strain as:

• strain_calc = e11.nx.nx + e12.nx.ny + ....

So 6 extra parameters (factors of 2 or not for e12). There might be a way to fit stress instead and then compute peak shifts taking into account some elastic constants, but this seems harder to get right.

Unknown for now is whether the eta/omega should be shifted once the eij are known? If so, in which directions?

You would probably want to separate this fit from the rest of the detector parameter refinement.

One application is better twotheta calc for peak assignments during indexing. Also would be nice to track the overall average strain.

[jonwright]

So this is probably going to come together with the new deformation gradient tensor business. A bit of thought remains to see what to do about eta/omega shifts? Should be 6 eij parameters to be fitted as a sample strain (assuming cell, wavelength and probably distance, are fixed).