Dear CCP4ers, <snip>
I think that characterize hydrogens as "transparent" to x-rays is somewhat misleading. There are plenty of examples where hydrogens are seen in ultrahigh resolution structures. Modern refinement programs all use riding hydrogens and it improves models. More precise statement is that at not-ultrahigh (non-atomic) resolution (i.e. worse than 1.2A) optical resolution is too low to distinguish two atoms separated by ~1A (typical value of X-H covalent bond).
</snip>
if one looks at atomic scattering curves, hydrogens contribute to X-ray scattering mainly at low resolution, which makes them so difficult to detect even at atomic resolution. I think, it is generally a good idea to include hydrogens as riding atoms in all crystallographic tasks, and especially during refinement, both because of their scattering contribution and because of their geometrical contribution, if the latter is considered in form of anti-bumping restraints and even more important in form of hydrogen bonds.
Best regards, Dirk. -- ******************************************************* Dirk Kostrewa Gene Center, A 5.07 Ludwig-Maximilians-University Feodor-Lynen-Str. 25 81377 Munich Germany Phone: +49-89-2180-76845 Fax: +49-89-2180-76999 E-mail: kostr...@genzentrum.lmu.de WWW: www.genzentrum.lmu.de *******************************************************
