On 09/01/2008, at 20.48, Anastassis Perrakis wrote:
I actually think that inaccurate cells are a big source of misery in many refinements. I have found the idea of WhatCheck to actually check your cell by looking at the projection of bond lengths of certain types along the cell axes most useful. I would hardly advocate to measure your cell that way, but going back to you data and looking at the cell again would be worth it.
However, cell parameters are quantities that have been experimentally determined, thus calculating them using the approach of WhatCheck is methodically incorrect. Furthermore, there is lots of scattering matter in the unit cell that is not accurately modelled by atoms.
Bond lengths and angles are only needed in macromolecular crystallography to overcome the poor data to parameter ratio. They are an artefact of life, so to speak. And as such, they are artificially tightly restrained to some ideal value. The data is normally not strong enough to tell us that a bond is _not_ ideal.
As any measurements, cell parameters have an uncertainty associated with them, so a better approach would be to propagate those errors in measurements in a proper statistical fashion.
Cheers, Morten -- Morten Kjeldgaard, asc. professor, MSc, PhD Department of Molecular Biology, Aarhus University Gustav Wieds Vej 10 C, DK-8000 Aarhus C, Denmark. Lab +45 89425026 * Mobile +45 51860147 * Fax +45 86123178 Home +45 86188180 * http://www.bioxray.dk/~mok
