Difference between revisions of "Phase problem"
From Online Dictionary of Crystallography
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| − | < | + | <font color="blue"> Problème des phases</font> (''Fr''). <font color="red">Phasenproblem</font> (''Ge''). <font color="black">Problema della fase</font> (''It''). <font color="purple">位相問題</font> (''Ja''). <font color="green">Problema de las fases</font> (''Sp''). |
== Definition == | == Definition == | ||
| − | Waves diffracted | + | Waves diffracted by a periodic distribution of simple scatterers obey [[Bragg's law]], which allows ready determination of [[interplanar spacing|interplanar distances]] and thus the easy recovery of a description of the crystal structure. Where the scattering objects are complex (''e''.''g.'' in molecular crystals) the diffracted radiation suffers a phase shift arising from the spatial distribution of individual scatterers. The amplitudes of the resulting [[structure factor]]s are directly derivable from the experimental measured intensities of the diffracted beams, but the phases are not. Without a knowledge of the phases, it is not possible to reconstruct the individual atomic positions. Estimating the phases is an essential step in successful [[structure determination]]. |
== Methods of solution == | == Methods of solution == | ||
| Line 11: | Line 11: | ||
* Guessing the structure | * Guessing the structure | ||
* [[Patterson methods]] | * [[Patterson methods]] | ||
| − | * [[ | + | * [[Molecular replacement]] |
| − | * [[ | + | * [[Direct methods]] |
| − | * [[ | + | * [[Multiple isomorphous replacement]] |
| − | * [[ | + | * [[Anomalous dispersion]] |
| + | * [[Charge flipping]] and other [[dual-space phase retrieval algorithms|dual-space retrieval algorithms]] | ||
| + | * [[VLD]] | ||
[[Category:structure determination]] | [[Category:structure determination]] | ||
Latest revision as of 09:46, 17 November 2017
Problème des phases (Fr). Phasenproblem (Ge). Problema della fase (It). 位相問題 (Ja). Problema de las fases (Sp).
Definition
Waves diffracted by a periodic distribution of simple scatterers obey Bragg's law, which allows ready determination of interplanar distances and thus the easy recovery of a description of the crystal structure. Where the scattering objects are complex (e.g. in molecular crystals) the diffracted radiation suffers a phase shift arising from the spatial distribution of individual scatterers. The amplitudes of the resulting structure factors are directly derivable from the experimental measured intensities of the diffracted beams, but the phases are not. Without a knowledge of the phases, it is not possible to reconstruct the individual atomic positions. Estimating the phases is an essential step in successful structure determination.
Methods of solution
A number of techniques exist to determine the phases associated with the structure factors. In general, they provide estimates of a set of phases, which are used to calculate a set of structure factors that may be compared with experimental observations. Among the techniques currently employed are:
- Guessing the structure
- Patterson methods
- Molecular replacement
- Direct methods
- Multiple isomorphous replacement
- Anomalous dispersion
- Charge flipping and other dual-space retrieval algorithms
- VLD