Charge-flipping algorithm in x-ray crystallography

I will post on results presented in Coherence-2007 conference soon, but this (or rather “last”) week’s entry is the echo of recent issue of Journal Club for Condendsed Matter Physics (formerly hosted by Lucent/Bell Labs), with a commentary by Simon Billinge on Charge-Flipping algorithm paper in Nature Materials by John Spence’s group at Arizona State.

It’s notable, especially since this is the first (to my knowledge) article that deals with rather technical developments in x-ray scattering field featured in JCCMP.

The basic idea is similar to phase-retrival algorithms, but deals with the loss of directions of H-K-L indices in powder diffraction data (as opposed to single-crystal diffraction where directions are preserved).

In addition to boring terms like “Error Reduction”, “Difference Map” or “Hybrid Input-Output” the field of phase retrieval has many colorful names, like “Charge Flipping”, “Shrink-Wrap”, “Shake and Bake” and “Magic man” (the last one is a Talladega-nights-inspired joke). In attempt to eliminate annoying phase vortices, some people use “voting” and “patching” approaches. Some historic and more recent papers on these topics will hopefully be featured here, but for now read the Wu et al paper “Ab initio phasing of X-ray powder diffraction patterns by charge flipping” Nature Materials 5 647 (2006).
Coming soon – how is phase retrieval in lensless imaging similar to solving a Sudoku puzzle or making multi-leg trip travel arrangements.

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