[ccp4bb]: SHELX-93 Vs SHELX-97 Q

["S. Ravichandran" <ravi@cmb2.saha.ernet.in>]

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Hello ALL,

In response to my following query

'  why does SHELX-93 outputs an e.s.d for bond-angles
   that involve Hydrogen, whereas SHELX-97 don't? and
   how reliable are they (in small molecule xtalography)? '

I got the following replies:-

=========================================================================
(1) From: Trixie Wagner <bw6f@unix.mail.virginia.edu>

Why do you have esd's for bond lengths involving hydrogen atoms if you
treat them as riding? The distance should be constant then, e.g. 0.96
A for a C-H distance in a methyl group. Regarding the esd's for angles,
you usually don't quote angles if the hydrogen involved has been refined 
with a riding model. All X-X-H angles are adjusted according to purely 
geometric criteria, which implies that e.g. in a methyl group they are 
all exactly equal. I assume esd's are not quoted because they are in 
general derived from the covariance matrix, which does not include 
coordinates from riding atoms. Their xyz values are always 'following' 
the coordinates from the non-hydrogen atom they are riding on. If I 
remember things correctly, they are added after each cycle of refinement 
and then used for the structure factor calculation.
--------------------------------------------------------------------------
(2) From: Gabriel Birrane <gbirrane@electra.bidmc.harvard.edu>

If you are using a 'riding model' then you will not be refining your
hydrogen positions. The hydrogens will be placed at their calculated
positions after each cycle of refinement. No extra parameters are added to
the refinement -> no esd information.
--------------------------------------------------------------------------
(3) From: George Sheldrick

The program SHELXL-93 calculated esds involving riding H-atom in a way
that was mathematically correct, assuming that the magnitude and
direction (but not position) of the X-H vector were exactly fixed; the
resulting esds thus depended on the combined scattering power of H and X
rather than just X. This gave rather small H-atom esds that confused
people, so I made the 97 version output zero esds for riding hydrogens.  If
you want realistic esds for hydrogens you have to refine them freely (not
recommended for proteins)!
=========================================================================

Thanks for the responses..

Cheers,
Ravi.