Can stacked_disk model support polydisperse parameters

[butler]

As discussed in #789, it is unclear how exactly the sigma_d works. Is it in fact a way of introducing polydispersity in core and layers (though exactly what layer then is the most polydisperse? or is it a proportional stretching of the cylinder? and if so how does that change the volume of the effective radius one should use when multiplying by an S(Q) between stacks?) or does in just introduce varying layers of water between the disks (though then it probably shouldn't be a Gaussian? having a cut-off when zero water exists? and again what is the implication for effective volume of disks?)

I would recommend that we remove polydispersity options from any of the parameters of the stacked disk model. Currently these are:

• radius
• the core thickness
• the layer thickness
• the n_stacking (number of disks in a stack)

I doubt that our infrastructure is handling these remotely correctly when sigma_d is none zero and unless we understand how sigma_d works I don't know how to figure out what and how to allow polydispersity. This will break with older versions which did allow polydispersity of course but still recommend we remove and when asked see if anybody out there can suggest how to do it… or show that it is correct to do as is.

[smk78]

Discuss at Code Camp

[smk78]

Need to formalise how we handle this kind of thing. Affects Save/Load of projects. If the underlying model changes between SasView versions then it should perhaps become a new model?

[butler]

That does raise an important philosophical question we have so far avoided having to confront: If a model is wrong and a change affected to correct it (thereby changing the behavior of the underlying model) should it be removed (assuming the correct model has been implemented as a new model as suggested above)? Keeping an incorrect model in the distribution seems totally wrong. On the other hand breaking old save states is also problematic. Then there is the question of it behaving differently from before which has been used as an excuse to not change things in the past. etc etc etc. This might be appropriate as a topic for discussion at a meeting?