Subject: Re: CCL:orbitals and reality

From: "Dr. N. SUKUMAR" <nagams{at}rpi.edu>
Date: Thu, 24 Feb 2005 1:42:44 EST

On Sat, 19 Feb 2005 15:13:59 -0800
 Eric Scerri wrote:

> 
> To my mind this is a rather disappointing attitude.  The emergence of 
> philosophical discussions among scientists is surely a sign of change 
> occurring and should be welcomed as a respite from the day-to-day 
> work and as an opportunity for practitioners from different parts of 
> the field to engage in fruitful discussion, more widely than in the 
> normal course of events.

The reason I, for one, lost interest in this thread was because I do not
care to carry on a religious argument for or against orbitals (experiment
be damned) with those who believe the rest of the world is deluded. Logical
discussion of philosophical points is, however, very much in order.

> 
> It seems to be that there are some rather formidable grounds for 
> believing in the real physical existence of electrons as opposed to 
> orbitals even though neither are directly observable...
> 
> Starting with J.J. Thomson in 1897, if not before, many properties of 
> electrons have been 'observed' such as its mass, charge and spin. 
> Would it not be a miracle if all these experiments pointed towards 
> the existence of the electron and yet there was no such particle (The 
> no miracle argument for realism).
> 
> ...
> 
> Now orbitals.
> By no stretch of the imagination is there the same mass of evidence 
> for the physical existence of orbitals!  
> 

OK, what about atoms in molecules? Do atoms "exist" within molecules? Do
functional groups "exist"? A physicist pre-Bader might have been tempted to
answer no. In conventional MO theory or wavefunction based QM, these
questions does not make sense. These theories donot make use of these
concepts. The concepts are undefined in the theory. But in Bader's theory
of atoms in molecules, these questions can definitively be answered in the
affirmative. Atoms and functional groups, as virial fragments, are
well-defined concepts or objects of the theory.

It has been argued here that because the theory of atoms in molecules makes
no conceptual use of orbitals, these objects are fictitious or superfluous.
Let me clarify that my own research work deals with the theory of atoms in
molecules. The flagship program of our group (RECON
http://www.drugmining.com) utilizes a database of transferable atom
equivalents, defined as virial fragments, for rapid high-throughput
descriptor generation, and a similar database of amino acid fragments for
proteins (Analytical Chemistry, 73, 5457-5461, 2001; J. Chem. Inf. Comput.
Sci. 42, 1347-1357, 2002). Personally I have little conceptual use for
orbitals. But orbitals gained the prominence they have today because
chemists borrowed these objects from physicists who were concerned with
explaining atomic spectra.

The fluid dynamical interpretation of quantum mechanics (which has a long
and illustrious history all the way back to Madelung and de Broglie) avoids
the use of orbitals and deals only with density and phase (or velocity
field). But how do we obtain excited state densities of many-electron
systems from ground state densities? Orbitals are a useful intermediate
concept. Does that bestow an objective reality upon them? For alternative
formalisms, see e.g. N. Sukumar and B. M. Deb,  Phase Associated with the
Single-Particle Density of Many-Electron Systems  Intl. J. Quantum Chem.
40, 501-510 (1991).

What about molecules themselves? Have molecules been observed? Most
chemists would find the question rather silly. However, one can have a
perfectly respectable theory of matter without employing the concept of
molecules and all experimental observations can be interpreted without
positing their existence: Molecules are just resonances in collision
theory. To a physicist, a molecule is simply a collision that takes too
long. So are molecules "real objects" or just useful (if approximate)
concepts? A semantic distinction.

On Wed, 23 Feb 2005 20:03:14 +0100 khinsen{at}cea.fr wrote:

> 
> The point I am trying to make is that "reality" has no place in a  
> scientific theory. The concept belongs to the realm of metaphysics...
> --

Exactly the point I too have been making.

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