The value of the KSOL solvent-model parameter describing
the ratio of the electron density in the bulk solvent to the
electron density in the molecular solute.
This is one of the two parameters (the other is
_refine.solvent_model_param_bsol) 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.