Commission on Crystallographic Computing

Charge flipping

Gabor Oszlanyi
Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, Budapest, Hungary

Charge flipping is a recently developed algorithm of ab initio structure determination [1]. It is essentially an iterative Fourier cycle that unconditionally modifies the calculated electron density and structure factors in dual spaces. The modifications have two different roles: they either act as constraints to limit the size of the search space, or as perturbations to avoid stagnation of the process. The name-giving step of charge flipping is a good example of a fine balance, changing the sign of electron density below a small positive threshold simultaneously forces positivity and introduces high-frequency perturbations. Such a working principle significantly differs from that of classical direct methods and offers complementary applications. The basic algorithm has some attractive properties: it solves the structure without utilizing atom types, chemical composition or any information on the space group symmetry.

The talk will give an overview of the basic principles, the algorithmic improvements of charge flipping, the choice of its parameters, software implementations and present successful applications in periodic and aperiodic crystallography.

The tutorial will help students to write their own code of ab initio structure determination and enjoy success within hours. Considering the algorithm's extreme simplicity and relying on current high-level programming languages (e.g. Matlab), this seems to be a realistic goal.

1. Oszlanyi, G. & Sueto A. (2008) Acta Cryst. A64, 123-134

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