Subject: A Perplexing AIM question

From: Gary Breton <[email-masked]>
Date: Thu, 18 Jul 2002 11:04:17 -0400

Hello all,

As there has been a rash of AIM questions as of late, I'd like to add one
more.

If I take ethane and run a minimization of the structure at the
RB3LYP/6-31G(d) level I obtain a perfectly nice structure as expected.  I
can take the Cartesian coordinates from the Gaussian output and create an
XYZ file that can be read within several programs, including the free
ViewerLite from Accelrys. All looks good, and I measure very reasonable C-C
bond lengths of 1.53 A, C-H bond of 1.10 and  a C-C-H angle of 111.4 (and I
say all this just to prove I can create an XYZ file properly!).

I then performed an AIM analysis on this structure within G98W (keywords:
output=wfn punch=archive aim).  Furthermore, I then took the wavefunction
generated and subjected it to a calculation of its critical points in the
(formerly free) program AIM2000.

Both G98W and AIM2000 generated exactly the same Cartesian coordinates for
the "attractors" and for the critical points on the attractor interaction
lines. (ok: so I can perform an AIM calculation properly, too!)

Finally, I simply lifted the cartesian coordinates from either of these two
output files and created a new XYZ file (I use ghost atoms (Bq) to define
the critical points). When I view the structure though, the C-C bond is now
2.89 A (!!) and the C-H bond 2.04 A but the C-C-H bond angle remains the
same.  (the same holds true if the Bq atoms are not included in the file).

As the "attractors" are expected to simply be the atomic nuclei, why are the
bond distances different from the minimized structure?  The answer is not
obvious to me.

Gary W. Breton
Chair and Associate Professor
Department of Chemistry
Berry College
PO Box 495016
Mount Berry, GA 30149

"There's a light at the
end of the tunnel, but it
may be an oncoming train"

---

From: Xavier ASSFELD <[email-masked]>
Date: Thu, 18 Jul 2002 18:19:37 +0200

> say all this just to prove I can create an XYZ file properly!).
...
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What about the conversion of Angstrom to Bohr...
1 Bohr = 0.529177 Angstrom
AIM2000 is working in a.u. system.

hope this helps...
-- 

                                      ...Xav

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