Subject: RE: crystal packing calculations and programs

From: Artem Masunov <[email-masked]>
Date: Sun, 16 Mar 1997 20:43:52 -0500 (EST)

From "Dr. Douglas A. Smith" <[email-masked]>:
>What are the options when one does not have the crystal structure available?
>Can one reasonably start from a high quality ab initio calculation on a
>single molecule (gas phase) and use that structure to pack a crystal?  If
>one does, what is the likelihood of getting the crystal structure correct?
>Or does one need to perform ab initio calculations in the periodic solid
>(e.g., CRYSTAL 95 calculations) before performing crystal packin(e.g.,
>CRYSTAL 95 calculations) before performing crystal packing?
>
>What packages are out there, commercial and "public domain" (free, QCPE,
>etc.) that can perform crystal packing calculations?  I would like to
>predict space group, density, lattice energy, etc.

Dear Dowg, 
	I just sent you a summary I posted on CCL a year ago, which has
all details. In general, there are two approaches to predict unknown
crystal structures: to build clusters and infinite agglomerates (1,2,3D) 
imposing some crystallographic symmetry elements (free package PROMET from
Gavezzotti), or to guess a lot of random structures and optimize them
using simulated annealing to reduce large number of local minima
(commercial packages from MSI, D.Williams, and J.Perlstein). Polymorph
predictor Cirrus2 from MSI is the most famous and most expensive. None of
the approaches guarantees correct result. If you are interested in
practical result, rather than in academic excersise, you will be better
off by analizing known crystal structures of analogs using Cambridge
Database.
	All programs above use empirical force fields. To parametrize a
force field takes a lot of efforts, and there are several systems
available for specific classes (peptides, carbohydrates etc.). It is very
dangerous to use only one crystal structure to parametrize FF, since not
all mutual orientations are taken into account.
	Ab initio calculations will NOT help. Except for atomic charges,
there is no easy way to calculate force field parameters from the
wavefunction.  
	You can not run CRYSTAL before you know the 3D-structure. Besides, 
the version being distributed now has no optimization implemented.
Semiempirical calculations are possible, but they are less reliable than
the good force field. 
	Hope this helps. If you need more details, please let me know.

Yours,

Artem

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