Re: [ccp4bb]: DM - NCS averaging

[gb10@globalphasing.com]

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According to David J. Schuller:
> 
> 
> i cannot recall anyone in the history of NCS averaging recommending 
> averaging to the exclusion of solvent flattening.
> 

Dear David,

     There is a good reason for that: you cannot average by NCS without 
flattening the solvent - by definition of NCS!

     If one wants nevertheless to use this perverse idea, i.e. preserve the 
solvent density features of the initial map while replacing the macromolecular 
density by its NCS averaged version within the envelope, then the degree of 
overdetermination of the equations decreases catastrophically, and leads to 
accumulating rubbish features in the solvent region. This was actually done 
by Rossmann et al. in 1978, on intermediate resolution maps of SBMV which 
started showing some tantalising density for "24 clearly visible phosphates" 
belonging to some putative RNA. When the work was extended to higher resolution
and common sense prevailed, the 24 phosphates were nowhere to be seen.

     Nowadays, further insights such as J.P. Abrahams's gamma-correction would 
make this mistake nearly impossible.


     With best wishes,

         Gerard.

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