The ab initio crystal structure solution of proteins by direct methods. IV. The use of the partial structure

[~/~]q A probabilistic formula originally designed for small molecules, which allows the recovery of the complete from a partial structure [Giacovazzo (1983). Acta Cryst. A39, 685-692], is reconsidered. Experimental tests show E~ that the formula is potentially able to estimate phases accurately provided 30--40% of the electron density is correctly located. The formula may be used for ref'ming the phase values obtained by isomorphous derivative ,, techniques as well as for extending the phasing process E".h to a resolution higher than the derivative resolution.