Re: CCL:Question on molecular structure and QM

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.
 >
 >
 >
 > --
 >
 >
 > Dr. Eric Scerri ,
 > UCLA,
 > Department of Chemistry & Biochemistry,
 > 607 Charles E. Young Drive East,
 > Los Angeles,  CA 90095-1569
 > USA
 >
 > E-mail :   scerri.-at-.chem.ucla.edu
 > tel:  310 206 7443
 > fax:  310 206 2061
 > Web Page:    http://www.chem.ucla.edu/dept/Faculty/scerri/index.html
 >
 > Editor  of  Foundations of Chemistry
 > http://www.kluweronline.com/issn/1386-4238
 >
 > Also see International Society for the Philosophy of Chemistry
 > http://www.georgetown.edu/earleyj/ISPC.html
 >
 >
 > -= This is automatically added to each message by the mailing script =-
 > To send e-mail to subscribers of CCL put the string CCL: on your Subject:
 > line
 > and send your message to:  CHEMISTRY.-at-.ccl.net
 >
 > Send your subscription/unsubscription requests to:
 > CHEMISTRY-REQUEST.-at-.ccl.net HOME Page: http://www.ccl.net   | Jobs Page:
 > http://www.ccl.net/jobs
 > If your mail is bouncing from CCL.NET domain send it to the maintainer:
 > Jan Labanowski,  jlabanow.-at-.nd.edu (read about it on CCL Home Page)
 > -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 >
 >
 >
 >
 --
             Brian Salter-Duke (Brian Duke) b_duke.-at-.octa4.net.au
 Honorary Fellow (Chemistry), Charles Darwin University, Darwin, Australia.
              Post: Box 1028, Humpty Doo, NT 0836, Australia.
 Phone 08-89881600.  Fax 08-89881302.  http://www.octa4.net.au/linden/brian/