Flat bulk-solvent model: obtaining optimal parameters.

A bulk-solvent correction is regularly used for macromolecular refinement. The flat model of the bulk solvent is considered to be the most reliable. It is shown that the standard procedure does not always result in the optimal values of the bulk-solvent correction parameters. A method to obtain the best values for parameters k(sol) and B(sol) of the flat-solvent model is discussed. The values of correctly determined parameters for crystallographic structures deposited in the Protein Data Bank are clustered around k(sol) = 0.35 e A(-3) and B(sol) = 46 A(2), which have a reasonable physical meaning. Such a distribution allows the use of these mean values of solvent parameters for many practical applications when refined parameters cannot be obtained, especially when an atomic model in the unit cell is not yet known.