On Fri, 23 Mar 2007, Edward Berry wrote: > I believe fft does not by default do this- only if you use the "fillin" > keyword? One place where it might be important is in density > modification/ molecular averaging. Molecular averaging can be seen > as a numerical solution of the MR equations, finding a density map > which (A) obeys the NCS/intercrystal symmetry and (B) yields the > observed F's upon Fourier transformation. Now if on each cycle > you set the missing F's to zero, you are requiring it to have > as part of (B) zero amplitude for the missing reflections, which is > more restrictive and incorrect. If instead you allow the missing F's > to "float", calculating them on each cycle from the previous map > using the fillin option, someone has shown (don't have the > reference handy at the moment) that the F's tend toward the true F's > (in the case that they weren't really missing but omitted as part > of the test). > > Ed
You have phase scatter plots in Acta Cryst. D51, 575-589 (1995) that show just that: the map inversion phases from NCS averaging tend toward the true phases. Since F's are phased quantities and since phases are more important than amplitudes, non random amplitudes plus non random phases (both from map inversion of averaged maps) lead to better electron density maps. Fred. -- Fred. Vellieux, esq. ================================= IBS J.-P. Ebel CEA CNRS UJF / LBM 41 rue Jules Horowitz 38027 Grenoble Cedex 01 France Tel: (+33) (0) 438789605 Fax: (+33) (0) 438785494 =================================
