Evaluation of macromolecular electron-density map quality using the correlation of local r.m.s. density

It has recently been shown that the standard deviation of local r.m.s. elec tron density is a good indicator of the presence of distinct regions of sol vent and protein in macromolecular electron-density maps here, it is demons trated that a complementary measure, the correlation of local r.m.s. densit y in adjacent regions on the unit cell, is also a good measure of the prese nce of distinct solvent and protein regions. The correlation of local r.m.s . density is essentially a measure of how contiguous the solvent (and prote in) regions are in the electron-density map. This statistic can be calculat ed in real space or in reciprocal space and has potential uses in evaluatio n of heavy-atom solutions in the MIR and MAD methods as well as for evaluat ion of trial phase sets in ab initio phasing procedures.