Abstract
The incorporation of the new peakness-enhancing fast Fourier transform compatible ipp procedure (ipp = inner-pixel preservation) into the recently published SM algorithm based on |ρ| [Rius (2020). Acta Cryst A76, 489-493] improves its phasing efficiency for larger crystal structures with atomic resolution data. Its effectiveness is clearly demonstrated via a collection of test crystal structures (taken from the Protein Data Bank) either starting from random phase values or by using the randomly shifted modulus function (a Patterson-type synthesis) as initial ρ estimate. It has been found that in the presence of medium scatterers (e.g. S or Cl atoms) crystal structures with 1500 × c atoms in the unit cell (c = number of centerings) can be routinely solved. In the presence of strong scatterers like Fe, Cu or Zn atoms this number increases to around 5000 × c atoms. The implementation of this strengthened SM algorithm is simple, since it only includes a few easy-to-adjust parameters.
Highlights
The novel SM;jj phasing function is rooted in the ZR originfree modulus sum function, a nearly 30 years-old directmethods phasing function (Rius, 1993)
It has been shown that the introduction of the new peaknessenhancing ipp procedure in the SM phase refinement algorithm significantly improves the algorithm efficiency for diffraction data at atomic resolution and, has been incorporated as the default option
The following rules could be established on the basis of the test calculations: (a) For very small light-atom crystal structures either Èrnd or ÈM0 phases can be used
Summary
The novel SM;jj phasing function is rooted in the ZR originfree modulus sum function, a nearly 30 years-old directmethods phasing function (Rius, 1993). The CK(È) = |CK(È)| exp[i K(È)] complex quantity is the Fourier transform of the |(È)| density function in terms of the È structure-factor phases to be refined. Their refinement is achieved by maximizing SM;jjðÈÞ through the iterative SM;jj fast Fourier transform (FFT) algorithm. One common feature of most iterative phase refinement algorithms working at atomic resolution and alternating between real- and reciprocal-space calculations is the density modification of the intermediate Fourier maps. Gaussians only at the previously determined peak positions (the rest being zero) This modification is part of Sheldrick’s intrinsic phasing procedure (Sheldrick, 2015) and allows the posterior application of the FFT algorithm.
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