When default EXPO2011 fails: strategies

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When default EXPO fails: strategies

       Crystal structures are solved by EXPO by default settings. In case of failure the user can modify the default pathway in order to improve the success rate. We give some suggestions to follow when Direct Methods fail.

Fourier syntheses usually suffer by truncation effects due to the limited data resolution (RES): the worst RES value, the less accurate is the electron density map. If RES is > 1 ? both the phasing and refinement process are difficult. The series termination errors are responsible for:

a) the presence of positive and negative ripples with decreasing amplitudes, located around each peak in the map;
b) the shift from the correct position and the broadening of each atomic peak, since the ripples overlap with other atomic peaks.

A new procedure (Altomare et al., 2008a, 2008b) has been introduced in EXPO, aiming at reducing the resolution bias effects and therefore at locating the peaks in more correct positions, eliminating false peaks and reducing the peak broadening.

The method has been successfully applied to a large set of experimental data that are expected to be a challenge for DM. The new approach is very effective particularly in case of organic structures (for them the resolution is usually far away the atomic one because of the rapid scattering factor decay) and enables EXPO to solve crystal structures previously not accessible by DM (Altomare et al., 2008a, 2008b). To apply RBM click on 'Refine' > 'Resolution Bias Modification (RBM, Direct Space)'



    

accessible by menu 'Restart' > 'Explore other trials'. Eventually you can also apply the RBM.

 

By the check button 'E-map' you can select only the e-map. Modify the number of cycles of Fourier recycling by the entry 'Number of cycle(s)'.

       Alternatively you can use the directive SET followed by the set number in the FOURIER command.

The number of options is quite large. The optional strategies regard the improvement of both the decomposition process for carrying out more reliable reflection intensities and the Direct Methods performances and the structure model. We give some other secondary strategies.

About the improvement of the decomposition process

About the optimization of Direct Methods procedure

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