Subject: CCL:orbitals and reality

From: Nicolas Ferré <nicolas.ferre * up.univ-mrs.fr>
Date: Mon, 31 Jan 2005 09:29:26 +0100

Dear CCL,

Regarding this discussion, I would like to make a parallel with 
classical mechanics. According to Newton's laws, a very useful concept 
raises: the trajectory, that is the ensemble of all the positions 
reachable by a point, given some initial conditions and some 
mathematical relationship of its movement.
Is this trajectory real ? Can we observe it ? No, we can only observe 
one position at the time. However we can reconstruct mentally and 
graphically this ensemble, and this is a very useful tool !
(By the way, a large part of our community commonly uses this tool to 
explore the configurational space of molecular structures through 
molecular dynamics simulations, without saying they are watching 
something real.)
For me, one-electron orbitals are like trajectories, I don't say "are 
trajectories" ;-) Only mathematical tools built from our mind because 
they are conceptually more understandable than a multi-electronic 
wavefunction. But they are very useful and I am very happy to use them.

-- 
  Dr. Nicolas Ferre'
  Laboratoire de Chimie Theorique et de Modelisation Moleculaire
  UMR 6517 - CNRS Universite' de Provence
  Case 521 - Faculte' de Saint-Jerome
  Av. Esc. Normandie Niemen
  13397 MARSEILLE Cedex 20 (FRANCE)
  Tel : (+33)4.91.28.27.33              Fax : (+33)4.91.28.87.58

  Please avoid sending me Word or PowerPoint attachments.
  See http://www.gnu.org/philosophy/no-word-attachments.html

---

From: Valentina VETERE <vetere <> esrf.fr>
Date: Mon, 31 Jan 2005 15:20:54 +0100

At 22:58 28/01/2005 -0800, Sengen Sun wrote:

>First, in response to Dr. N. Sukumar s comments, I
>feel that the claim for the observation of HOMO in the
>current Nature s paper is not acceptable whatever you
>say. The simplest reason before anything else is that
>all their mathematical schemes leading to their final
>claim are based on their assumption that one electron
>has to be confined to HOMO. We all know this is not
>true in the reality and in the context of QM.

Sorry I'm still in trouble here..can you please make me understand why an 
electron can not be localized in a HOMO. Is the reason that outside the 
orbital picture there is a probability different from zero to found the 
electron  or are you thinking about a multiconfigurational nature for the 
atoms?

Thank you
Valentina Vetere

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From: "Dr. N. SUKUMAR" <nagams{at}rpi.edu>
Date: Mon, 31 Jan 2005 16:03:30 EST

> molecular dynamics simulations, without saying they are watching 

Exactly! And this "ureality" of trajectories (or I would say, their
approximate nature -- due to the uncertainity principle) does not preclude
our ability to detect them experimentally, as in a Wilson cloud chamber.

If trajectories "exist" in the classsical approximation, then orbitals
"exist" in the independent-particle approximation. In both cases, we can
design and perform experiments to test the range of validity of these
approximations.

I am not claiming that the results of any particular experiment are valid
or otherwise, but the whole idea of declaring that the experiment has to be
wrong because we read in some undergraduate textbook that it cannot be done
is reminescent of the wey biology was supposedly practised in medieval
Europe: when there was a scholarly debate as to how many teeth a horse has,
with various scholars citing different chapters and verses of ancient Greek
texts, one upstart had the temerity to go out into the stables and bring in
a horse for experimental measurement!

Dr. N. Sukumar
Center for Biotechnology and Interdisciplinary Studies
Rensselaer Polytechnic Institute

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From: Eric Bennett <ericb/at/pobox.com>
Date: Mon, 31 Jan 2005 14:13:05 -0600

> molecular dynamics simulations, without saying they are watching 

Maybe I'm just picking apart an analogy by taking it too far, but...

You can certainly construct an experiment to capture a picture of a 
trajectory, without mental/graphical reconstruction.

Your analogy here is trajectory=orbital, and other people in this 
discussion have argued that any resemblance between electron density and 
the shape of orbitals is purely coincidental.  If I take a time lapse 
photo of your Newtonian object that shows a smear of the object that 
looks like the object's trajectory, is the resemblance just 
coincidence?  I don't think so.  It's reproduceable under many 
conditions with many objects.

Another analogy similar to yours but more related to the current 
discussion is that detecting electron density by various means (such as 
x-ray crystallography) often involves time averaging, the same as taking 
a time lapse photo of your classical object.   I think many people would 
argue that such experimental methods are, in fact, "watching something 
real" despite the time averaging.

As I see it, the debate in this thread is: are there any tricks 
analogous to time lapse photos that would allow you to capture a picture 
of an orbital?  Or are you just capturing a picture of electron density 
(or change in density) which only has a coincidental resemblance to an 
orbital... a resemblance that would NOT be there if you repeat the 
technique on a different molecule?  The analogy to trajectories doesn't 
seem helpful in resolving that disagreement, but perhaps I'm missing 
something.

-Eric