The value of the BSOL solvent-model parameter describing
the average isotropic displacement parameter of disordered
solvent atoms.
This is one of the two parameters (the other is
_refine.solvent_model_param_ksol) in Tronrud's method of
modelling the contribution of bulk solvent to the
scattering. The standard scale factor is modified according
to the expression
k0 exp(-B0 * s2)[1-KSOL * exp(-BSOL * s2)]
where k0 and B0 are the scale factors for the protein.
Ref: Tronrud, D. E. (1997). Methods Enzymol. 277, 243-268.