There was a good answer to this question on the CCP4BB recently (okt 11, Dale Tronrud), see below. The FFT chapter in the international tables by Bricogne is of course also a good thing to read. HTH Peter ---------------------------------------------------------------------------------------------------------------- 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. ---------------------------------------------------------------------------------------------------------------- 2007/10/29, Terry Lang :

Hey Everyone,

I have been told by several crystallographers that the general rule of thumb for grid spacing when generating a map is one third the resolution (eg If your crystal structure has a resolution of 1.2 A, your grid spacing should be 0.4). Is there a mathematical reason why this ratio should be maintained or is this simply a holdover from slower computers with lower memory? More importantly for my application, is there a lower limit on how fine the grid spacing can be? In other words, is there a lower limit at which one is oversampling the data for a particular resolution? Any information and/or references would be greatly appreciated.

Sincerely, Terry

-- P. Therese Lang Post Doc Alber Lab, UC Berkeley _______________________________________________ phenixbb mailing list [email protected] http://www.phenix-online.org/mailman/listinfo/phenixbb