add Boltzmann distribution

[dirk]

To describe the orientation behaviour of magnetic particles with moment mu (in terms of Bohr magneton muB) in a magnetic field of strength mu0 H (in T), usually the Boltzmann propability distribution P(x)=L exp(L x)/exp(-L)/exp(2L-1) is used. Here x=Cos(theta), where theta is the angle between the magnetic easy axis of the magnetic moment with respect to the magnetic field direction. The so-called Langevin factor L= mu0 H mu muB/kb T.

[dirk]

To make it similiar with the other Sasview distribution, the plan is now to parametrise the Boltzmann like P(x) = exp( -|x - c|/sigma) where sigma=k T/E is the inverse Boltzmann factor. So for an aligning system at high field with large Zeeman energy, i.e. Em>k T, sigma will tend 0, as it is usually defined.

[dirk]

In a5a12caaceba1c450d16a4bf8a7fc30ceab60a04/sasmodels:

implementation of Boltzmann distribution refs #1018

[richardh]

Why is the  sasmodels branch - "Boltzmann" - associated with this ticket still not pulled?

I think that this branch also has a "uniform" distribution as well as the rectangular one, see #764

Richard

[richardh]

in sasmodels branch Boltzmann if I save project then load it again, a “uniform” distribution fails to load back in, gives error messages

2018-01-16 17:24:50 : Error: This model state has missing or outdated information.

Traceback (most recent call last):

File "C:\sasview42\sasview\src\sas\sasgui\perspectives\fitting\fitpage.py", line 2036, in reset_page

self.reset_page_helper(state)

File "C:\sasview42\sasview\src\sas\sasgui\perspectives\fitting\basepage.py", line 1283, in reset_page_helper

self._reset_page_disp_helper(state)

File "C:\sasview42\sasview\src\sas\sasgui\perspectives\fitting\basepage.py", line 1375, in _reset_page_disp_helper

disp_model.set_weights(self.values[key],

KeyError: 'theta.width'

2018-01-16 17:24:50 : Completed loading.

The Boltzmann distribution seems to work OK, so looks like it is Paul K’s recent (good) idea to do away with Nsigs for the Uniform distribution is upsetting the loader.

[butler]

This change was merged a while back but apparently the ticket was not closed. Doing so now.