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.