On Thu, Jan 12, 2012 at 8:11 AM, Pavel Afonine <[log in to unmask]> 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
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