On Thu, Jan 12, 2012 at 8:11 AM, Pavel Afonine <[email protected]> wrote: > >> Who needs hydrogens? > > > may be you need to read this (for example): > > http://www.phenix-online.org/papers/dz5209_reprint.pdf >
While this reference is useful, it neglects the role of prior chemical forces (vdW and electrostatics, for example) in positioning hydrogen atoms. The X-ray/neutron data is often not sufficient to uniquely define an atomic position (hydrogen or otherwise), which can be especially problematic for atoms with several degrees of freedom, like water or a hydroxyl hydrogen. Force fields have come a long way in defining these forces with reasonable chemical accuracy in the past 10 years, and there is work to show this does benefit X-ray/neutron refinement (e.g. http://dx.doi.org/10.1016/j.str.2011.01.015) - suggesting its worthwhile to include this information in X-ray target functions. At the very least, it should not be left out of the discussion, especially when hydrogen atoms are concerned!!! Regards, Tim
