SFALL is calculating structure factors from the map you supplied,
so there is only one grid, the one you used when you created the
map in NCSMASK.

    The choice of sampling rates for maps to be Fourier transformed is
a deep topic.  The mathematical law is that you have to sample the
map at, at least, twice the frequency of the highest Fourier component
in the map.  This is, unfortunately, often misinterpreted as twice
the frequency of the highest component you are interested in.

    The fact that you are interested in, say, 2A structure factors
has nothing to do with the calculation of the Fourier transform of
your map.  All that matters is the frequencies that were present in
your map before you sampled it on the grid.

    Ten Eyck, (1977) Acta Cryst A33, 800-804 has a discussion of this
and provided the classic solution to this problem when the map to be
transformed is a calculated electron density map.  I presume you have
an NCS averaged map and the required interpolations introduce needs
of their own that are significant.  Gerard Bricogne has written on that
topic, also back in the 1970's, but I don't have the reference at hand.

    The manual for your NCS averaging program should give you guidance
on the choice of sampling rate based on its interpolation method.  If
you are not even sampling at twice the resolution you are interested
in you are sampling way too coarsely.

    All FFT based structure factor programs require that the sampling
rates along each axis be even.  They may have other required factors
depending on the space group, but they will be happy to inform you
if you make a choice it doesn't like.  They are also more efficient
when the prime factors of the sampling rates are small numbers.  Try
to stick with multiples of 2,3, and 5 if possible.

    Since the program has no way of knowing the highest resolution
component actually in the map before you sampled it on your grid,
it assumes that the map contains no components of higher resolution
then you asked it to produce.  All FFT programs will fail if you
sample your map courser than twice that frequency, as SFALL did for
you.

    That does NOT mean that twice the frequency you are interested in
is sufficient.  You MUST read your NCS averaging program's documentation
and if that doesn't tell you, complain the the program's author, and
read Gerard's papers on the matter.  NCS averaging a map that is only
sampled at twice the rate you are interested in will not be a useful
way to spend your time.

Dale Tronrud

whittle wrote:
> Hi:
> 
> I am trying to generate structure factors from a mask/map that I made
> with NCSMASK. I get the following error message in SFALL:
> 
> The program run with command: sfall HKLOUT
> "/tmp/whittle/109_5_7_1_mtz.tmp" MAPIN
> "/home/whittle/projects/109_5/CCP4/center_50.msk" 
> has failed with error message
>  SFALL:   Grid too small- NZ must be > 2*Lmax+1
> 
> Which grid is this referring to? The grid used by SFALL or by NCSMASK
> when I initially generated the map? How does one choose an appropriate
> grid and extent for these programs?
> 
> Thanks for your help!
> --James