[cctbxbb] Finding neighbouring atoms using pair_asu_table

[Nathaniel Echols]

On Wed, Apr 17, 2013 at 10:40 AM, Lukas Reck <reckl at tcd.ie> wrote:
> I'm trying to use the cctbx python libraries to read in a large number
> of crystal structures and extract some geometric information on the
> coordination environment of certain atom types in them. I've been
> following the instructions from the 8/2004 CompComm Newsletter No. 4
> at <http://www.iucr.org/__data/assets/pdf_file/0003/6384/iucrcompcomm_aug2004.pdf>,
> pages 22-28 (is that still up to date?) but I cannot seem to get at
> the correct symmetry elements for each neighbouring atom in the
> coordination environment of some atom of interest.
> ...
> As you can see from the coordinates, the neighbouring atom was
> correctly identified, but the corresponding symmetry as returned by
> get_rt_mx transforms it into a symmetry equivalent that is very far
> away.

To be honest I have never fully understood how this works, but for
some reason it is necessary to perform an additional operation on the
symmetry operator obtained by calling asu_mappings.get_rt_mx(j_seq,
j_sym):

rt_mx_i_inv = asu_mappings.get_rt_mx(scatterer_index, 0)
rt_mx = rt_mx_i_inv.multiply(asu_mappings.get_rt_mx(j_seq, j_sym))
site_symm = rt_mx * scatterers[j_seq].site

Hopefully someone who actually understands symmetry (or our
implementation of it, rather) can explain further.  At any rate, I've
attached a script showing representative usage starting from your
example, using a structure in the PDB.  (I'm displaying Cartesian
coordinates because I'm a biologist, not a chemist, but all of the
symmetry operations use fractional coordinates.)

We should probably include something like this in cctbx/examples or
the like (I didn't see anything similar there), but I'm not sure what
the preferred usage of the ASU mappings is - I do something slightly
different in some of my code (mmtbx/ions/__init__.py,
"find_nearby_atoms" method), but I was mostly just guessing when I
wrote that.  (Either form works, at least.)

-Nat
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