Re: [cctbxbb] Structure factor calculation weirdness

Graeme, Just as a general ballpark figure, for protein work the difference between approximate FFT structure factors and the exact direct sum figures is on the order of 1%. Nick On Fri, Dec 7, 2012 at 8:29 AM, wrote:

Bing! Bet Nat's hit this in one.

There may be a resolution dependence for 'F' but F001 (say) should not change, and should not go from 0 to non-0 no matter what. Direct summation much better here...

Presumably for a protein structure the effects of gridding will also be much less significant.

Thanks,

Graeme ________________________________________ From: [email protected] [[email protected]] on behalf of Nathaniel Echols [[email protected]] Sent: 07 December 2012 15:46 To: cctbx mailing list Subject: Re: [cctbxbb] Structure factor calculation weirdness

On Fri, Dec 7, 2012 at 7:36 AM, wrote:

...

Some confusion here at diamond – it seems that the amplitude of F(hkl) depends on dmin

Yes, because it's using the FFT method, which means it first does map sampling on a grid whose dimensions will partly depend on the resolution. I'm not sure if there is resolution dependence for the scattering factors (I thought so, but I don't really know details), but I tried using direct summation and the resolution effect disappears.

f_calc = structure.structure_factors( algorithm="direct", d_min = dmin).f_calc()

For your toy example the speed difference isn't noticeable - for a protein structure it will be considerable.

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