He Hongxing, Su Wu Pei
Department of Physics and Texas Center for Superconductivity, University of Houston, Houston, Texas 77204, USA.
Acta Crystallogr A Found Adv. 2018 Jan 1;74(Pt 1):36-43. doi: 10.1107/S205327331701436X.
In an iterative projection algorithm proposed for ab initio phasing, the error metrics typically exhibit little improvement until a sharp decrease takes place as the iteration converges to the correct high-resolution structure. Related to that is the small convergence probability for certain structures. As a remedy, a variable weighting scheme on the diffraction data is proposed. It focuses on phasing low- and medium-resolution data first. The weighting shifts to incorporate more high-resolution reflections when the iteration proceeds. It is found that the precipitous drop in error metrics is replaced by a less dramatic drop at an earlier stage of the iteration. It seems that once a good configuration is formed at medium resolution, convergence towards the correct high-resolution structure is almost guaranteed. The original problem of phasing all diffraction data at once is reduced to a much more manageable one due to the dramatically smaller number of reflections involved. As a result, the success rate is significantly enhanced and the speed of convergence is raised. This is illustrated by applying the new algorithm to several structures, some of which are very difficult to solve without data weighting.
在一种为从头开始相位确定而提出的迭代投影算法中，误差度量通常在迭代收敛到正确的高分辨率结构之前几乎没有改善，直到出现急剧下降。与此相关的是某些结构的收敛概率较小。作为一种补救措施，提出了一种对衍射数据的可变加权方案。它首先专注于对低分辨率和中等分辨率数据进行相位确定。当迭代进行时，加权会转移以纳入更多高分辨率反射。结果发现，误差度量的急剧下降被迭代早期不太显著的下降所取代。似乎一旦在中等分辨率下形成了良好的构型，几乎就可以保证朝着正确的高分辨率结构收敛。由于所涉及的反射数量大幅减少，一次性对所有衍射数据进行相位确定的原始问题被简化为一个更易于管理的问题。结果，成功率显著提高，收敛速度也加快了。通过将新算法应用于几种结构来说明这一点，其中一些结构在没有数据加权的情况下很难求解。
