One of the early references for mFo- nFc is: Vijayan, *Acta Cryst.* (1980). A*36*, 295-298
You may also like to read the book on Fourier transforms in crystallography by Ramachandran and Srinivasan. Shekhar On Fri, Jul 30, 2010 at 4:25 AM, Pavel Afonine <pafon...@lbl.gov> wrote: > Hi Ian, > > please correct me if I'm wrong in what I'm writing below... > > My reasoning for writing it like this > > > 2Fo-Fc = Fo + (Fo-Fc) > > is: > > 1) the map (Fo, Pcalc) shows density for missing atoms at half size > (approximately) > 2) the map (Fo-Fc, Pcalc) shows density for missing atoms at half size > (approximately) > 3) then the map (2Fo-Fc, Pcalc) shows density for missing atoms at full > size (approximately), and this is why this map is preferred over (Fo, > Pcalc). > > And maximum-likelihood weighted map 2mFo-DFc is even better since in > addition it is expected to be less model biased. > > This was my "rationale" to write 2Fo-Fc = Fo + (Fo-Fc) and not Fc + > 2(Fo-Fc) . > > Pavel. > > > > On 7/29/10 2:38 PM, Ian Tickle wrote: > >> On Thu, Jul 29, 2010 at 8:25 PM, Pavel Afonine<pafon...@lbl.gov> wrote: >> >>> Speaking of 3fo2fc or 5fo3fc, ... etc maps (see classic works on this >>> published 30+ years ago), I guess the main rationale for using them in >>> those >>> cases arises from the facts that >>> >>> 2Fo-Fc = Fo + (Fo-Fc), >>> 3Fo-2Fc = Fo +2(Fo-Fc) >>> >>> To be precise, it is actually >>> >>> 2mFo-DFc for acentric reflections >>> and mFo for centric reflections >>> >> I prefer to think of it rather as >> >> 2mFo - DFc = DFc + 2(mFo-DFc) for acentrics and >> mFo = DFc + (mFo-DFc) for centrics. >> >> Then it also becomes clear that to be consistent the corresponding >> difference map coefficients should be 2(mFo-DFc) for acentrics and >> (mFo-DFc) for centrics. >> >> Cheers >> >> -- Ian >> > -- Shekhar C. Mande Centre for DNA Fingerprinting and Diagnostics Bldg.7, Gruhakalpa 5-4-399/B, Nampally Hyderabad- 500001 INDIA Phone: +91-40-24749401 Fax: +91-40-24749448
