BE Polyelectrolyte errors

[butler]

Sylvain Prevost sent the following in November 2017 (subsequently lost and refound) regarding problems with this model. He believes there are two problems:

• In the python code: concentration is rescaled from mol/dm³ to 1/ų only for monomer concentration, not for salt concentration. Further, polymer concentration is probably a misnomer for monomeric units concentration

• In the docs:
  • The definition of r₀² is wrong, { b/√(48 π L_b) } should be in the numerator, not the denominator: r₀² = b / [ α √(C_a 48 π L_b) ]
  • in the main equation, it should read r₀⁴, not r₀²
  • in the docs, the conversion of concentration from mol/dm³ to 1/ų should be explicitly indicated before any equation
  • for consistency, in the main equation, it should read (q² - 12 h/N_A C_a / b²) where N_A is Avogadro's constant (as h is supposedly in ų/mol, and assuming C_a has been converted to 1/ų

[butler]

going through the math (is all in python) it is clear that indeed:

• r₀² is calculated as described above - documentation needs to be fixed
• The code calculates r_squared**2 which indeed is r₀⁴ - documentation should be fixed
• the virial parameter h is supposed to be given in ų NOT L/mol - equation in docs is ok BUT parameter units needs to be changed
• Indeed, units would seem to indicate that the salt concentration should also be converted to 1/ų. Looking at the IGOR code this was originally drawn from, it is interesting to note that indeed Ca and Csa are both calculated … but then only Ca is used as here. This may be a mistake that goes way back. This needs further checking.

[butler]

At this point all remaining tickets should either be blockers or critical. Critical would be those that do not stop the actual release but need to be dealt with soon after such as updating the marketplace. Tickets such as this are blockers in that if not fixed the docs need to have an appropriate warning in them for this release. Once that is fixed the ticket moves to 4.3 if it is not closed.

[GitHub <noreply@…>]

In a5bcd61ecf04134efa21afe21e10363f33f72063/sasmodels:

Merge pull request #82 from SasView?/ticket-1155BE_PolyElectrolyte

Fix docs and code for be_polyelectrolyte.py

Closes #1155
Will open a new ticket that validation is still required.