CRUNCH: a phase refinement program for high-resolution macromolecular structures

A phase refinement program for high-resolution macromolecular structures is described. The mean phase error of an initial structural solution can ofte n be as large as 60 degrees prior to attempting a refinement, but the CRUNC H program procedure may allow one to reduce this phase error to 10 or 20 de grees for all data prior to actually inspecting any map to see how well it may fit a structural model. The procedure is similar to the real/reciprocal space refinement loop incorporated in the half-baked algorithm described b y Sheldrick [Sheldrick & Gould (1995). Acta Cryst. B51, 423-431.] Refinemen t results are presented for nine different structures which diffract to bet ter than 1.2 Angstrom resolution. The program and test data are available v ia anonymous ftp ( nexus.hwi.buffalo.edu).