Subject: crystal packing calculations and programs

From: Robert Gdanitz <[email-masked]>
Date: Wed, 5 Feb 1997 15:47:55 +0100 (MEZ)

Dear Colleagues:
 
Concerning the discussion of crystal structure prediction, I would like point
out a few things that might be of interest for scientists working in the topic:

* My review called "Ab-Initio Prediction of Possible Molecular Crystal 
  Structures" has just been published by Wiley & Sons in the book "Theoretical 
  Aspects and Computer Modeling," edited by A. Gavezzotti.

  It also describes the algorithm of my paper "Prediction of molecular crystal 
  structures by Monte Carlo simulated annealing without reference to diffraction
  data," Chem. Phys. Lett. 190 (1992) 391, in a more didactical way. In "Are 
  Crystal Structures Predictable?," Acc. Chem. Res. 27 (1994) 309, Gavezzotti 
  assesses the computer software used in this work and later (mainly by 
  Karfunkel), as "presumably the best available at the moment".

* I have originally written this software for Karplus' "CHARMm", but I recently 
  generated a separate Fortran Code, which is to be interfaced to an arbitrary
  molecular modeling program. This code now is being used by Hoechst/Germany. 
  My original software (see above) meanwhile has found its way from Ciba-Geigy/
  Switzerland into Cerius2 of MSI (without making me rich - contrary to a 
  royalty agreement with MSI/Cambridge).

* To the question of predicting the space group, I would like to point out, that
  already in the first paper of my method (see above), it has been demonstrated,
  how the space group can be determined by assuming no symmetry and varying Z. 
  However, because owing to increasing problems with the optimization of each 
  Monte-Carlo trial structure, this approach is, in general, likely to be less 
  efficient than simply scanning all possible space groups. First of all there 
  is only a finite number (230 including the enantiomorphic pairs) of which only
  a small subset is found for organic molecular crystals, the no. most probable
  groups being about 20. Additionally there is the problem of the possibility of
  varying numbers of molecules in the asymmetric unit, which may cause 
  additional problems (see e.g. the above book for more details).

Anyway, since computers presently can expected to become 10 times faster every 
5 years, there is hope, that problems that now appear to be much too time 
consuming to be solvable, become tractable in the near future. In 1989, when I 
started to develop the method mentioned above, I barely could get enough
CPU-time - I do not believe I would have made it 5 years earlier!

Robert
-- 
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