Crystal structure prediction techniques used

A number of computational methods are frequently used in crystal structure prediction programs. To assess the differences between the programs, we list the most important techniques commonly used and mention their strengths and weaknesses. 

As can be immediately visualized, the predicted powder pattern for 01-126 (Fig. 7.4 middle) represents the predicted powder pattern of polymorph 1 (Fig. 7.4 top) better than what the predicted powder pattern of 01-127 (Fig. 7.4 bottom) does, especially noticeable at 20 -21°. In addition to comparing predicted powder pattern from an SXRD-determined structure, it is also common and useful to directly compare predicted powder patterns to experimentally determined powder patterns. This is a very valuable technique when there is no solution from single crystals available. Such an approach allows for rapid screening of several hundreds of predicted structural candidates to match an experimental powder pattern. Ideally the structure, both the molecular and crystal cell, from such a predicted structural crystal structure may be used as an input for further refinement of the powder pattern in a second iteration to completely match powder pattern intensities and peak positions. 

Woodley SM (2004) Prediction of Crystal Structures Using Evolutionary Algorithms and Related Techniques 110 95-132... 

While it is very easy, when one knows the structure of the crystal and the wavelength of the rays, to predict the diffraction pattern, it is quite another matter to deduce the crystal structure in all Its details from the observed pattern and the known wavelength. The first step is lo determine the spacing of the atomic planes from the Bragg equation, and hence the dimensions of the unit cell. Any special symmetry of the space group of the structure will be apparent from space group extinction. A Irial analysis may (hen solve the structure, or it may be necessary to measure the structure factors and try to find the phases or a Fourier synthesis. Various techniques can be used, such as the F2 series, the heavy atom, the isomorphous series, anomalous atomic scattering, expansion of the crystal and other methods. 

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