Subject: CCL:Question on molecular structure and QM

From: Eric Scerri <scerri..at..chem.ucla.edu>
Date: Sat, 10 Jul 2004 16:31:14 -0700

Thanks for your interesting response Brian and all others who have 
responded off-list.  Please keep your replies coming.

Your point about structure being reduced to physics, if not pure QM 
with no B-O, is well taken.  I see little point in trying to 
demarcate between QM and physics in this context.  For example the 
status of the Pauli Exclusion principle vis a vis QM has never been 
quite settled.  And yet nobody would claim that introducing the spin 
quantum number to Schrodinger's non-relativistic model represents a 
problem with the reduction of chemistry to QM.

Here is another thought on the subject which I hope is not too 
trivial.  The importation of B-O allows the molecule to have some 
structure but of course does not in itself dictate the details of the 
molecular structure.  It just allows the calculation to get off the 
ground, so to speak.  It is not as if the calculations are 
semi-empirical as when certain experimental data are introduced 
although they are semi empirical in the broad sense of saying that 
nuclei are much heavier than electrons.

What is behind this thinking is that there are many claims to a lack 
of full reduction of chemical phenomena to QM (or physics).  I am 
trying to weight up whether the molecular shape question is more or 
less serious than say the inability to strictly predict chemical 
periodicity without using the experimental order of orbital filling 
(n + l rule).  Also discussed in the Lowdin volume and something you 
and I have talked about.

regards,
eric

>On Thu, Jul 08, 2004 at 11:59:33PM -0700, Eric Scerri wrote:
>  >
>>
>>  The following is a somewhat general philosophical question.
>>  I would very much appreciate the comments of computational chemists.
>>
>>  In the philosophy of chemistry literature much has been made of the
>>  fact that molecular structure cannot be strictly deduced from QM.  I
>>  am referring to the work of Guy Wooley in the UK and Hans Primas at
>>  the ETH.
>>
>>  The basis of this claim is that structure comes after applying the
>>  Born-Oppenheimer approximation to fix the nuclei.  These authors
>>  claim that in this sense molecular structure has not been "reduced"
>>  to QM.
>>
>>  They point to the fact that the Hamiltonians for two isomers of an
>>  organic compound, for example, are identical in the absence of the
>>  B-O approximation and they conclude that a strictly QM calculation
>>  cannot therefore distinguish between the two structures in question.
>>
>>  I would be happy to pull some references together for anyone
>>  interested in studying the original articles.  But I just wanted to
>>  hear some general reactions to such claims.
>
>I think your conclusions are essentially correct. It is what we do with
>them that is the problem.
>
>A key point that I like to demonstrate this and which I owe to Brian
>Sutcliffe is the point that quantum mechanics says that all identical
>particles are indistinguishable. The Born-Oppenheimer approximation
>changes a system where all electrons are indisdinquishable and all like
>nuclei are indistinguishable to one of indistinquishable electrons in
>the field of distiquishable like nuclei. For example CO2 with two O-16
>isotopes has two indisquishable O nuclei and we teach students that this
>leads to alternate lines in the rotational fine structure of the IR
>spectrum to be missing. However the standard quantum chemistry methods
>have the two O nuclei as distinguishable. There is a lot that has to be
>worked out here. See Brian Surcliffe's article in the Lowdin memorial
>volume.
>
>Molecular structure is not as of now reducible to pure quantum mechanics
>but this does not mean it is not reducible to physics since the
>Born-Oppenheimer approximation is part of physics and the calculations
>agree with experiment.
>
>I am not so clear about the point about isomers. A pure non-B-O
>calculation would lead to all isomers and the dynamics of
>interconversion I think. Maybe people who do non-B-O calculations on
>small sytems could comment.
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