Re: Ab initio & Monte Carlo

Armel Le Bail ( armel@fluo.univ-lemans.fr )
Mon, 09 Feb 1998 09:48:57 +0100

Nita Dragoe wrote:
>I have, again, a question related to classifications. Can you clarify what do
>you understand by "independent atoms" in the case of "guess" structures ?

"Independent atoms" is a general definition in crystallography : those atoms
having each a different set of x,y,z coordinates. Even for a known molecule,
refinements are preferred without constraints. Of course, molecule shape
constraints may reduce the free parameter number at the "guess" stage.

>Take, for instance, the case of a coordination compound with 1-10
>phenanthroline, like Ni(o-phen)3 Cl2.
>Since we know the structure of phenanthroline I don't think that we need 14*3
>positions for location of the non-hydrogen atoms of this molecules (the
guess is
>then limited at fewer positions.. ). Is not the same case for these thienyl
>structures ?

You are right, of course. So that a structure with 30 independent atoms
is finally not more complex than a structure with one atom - if you have
just to find the orientation of one molecule in a cell : a kind of trivial
case. The classification of ab initio structure determination shows two
top lists. One with the largest number of independent atoms (C1) and
one with the largest number of atoms located by Patterson or direct
methods (C2) which were sufficient for completing then the structure by
Fourier difference. The problem is : should I consider C1 or the
number of independent molecules (M1) in case of "guess" structures when
listing the most complex structures ever determined ?:-).

Armel Le Bail - Universite du Maine - Laboratoire des Fluorures
CNRS UPRESA 6010 - Avenue O. Messiaen - 72085 Le Mans cedex 9 - France
Web : http://fluo.univ-lemans.fr:8001/