BULK <k_sol B_sol> :

This is an attempt to incorporate a primitive bulk-solvent correction algorithm in QS. The idea is to use the exponential scaling model to down-scale the amplitudes of the low resolution data. Unfortunately, it is impossible to do a proper job since that would entail several rounds of non-linear scale refinement at each step (see review by Tronrud, D. E. (1997), Methods Enzymol., 277, 306-319.). So, what we do instead is to keep the values of the two solvent parameters ( k_sol, B_sol) constant and equal to either the default values (when no argument is given next to the BULK keyword), or to the values given by the user. Now, about the values : the usual assumption is that k_sol is equal to the ratio of the mean electron densities of the protein and solvent components, and B_sol is related to the sharpness of the boundary between them. Unfortunately, things are not that simple : when I checked the PDB-deposited structures with respect to the distribution of the reported values for these two parameters the results were rather disappointing (you can see the distribution obtained from a paper abstract at my homepage http://www.mbg.duth.gr/~glykos/). I believe that down-scaling the low resolution terms with the default values can not make things worse than they are without any correction, so you may as well decide to include this keyword right from the beginning (but note that because it has not been extensively tested it is not the default).