Subject: RE: time-dependent and time-independent QM

From: "Shobe, David" <dshobe)at(sud-chemieinc.com>
Date: Mon, 12 Jul 2004 10:57:16 -0400

A possibly related question: there are two versions of the Hamiltonian, the time-dependent and the time-independent.  The former represents the evolution of a dynamic system; the latter represents the steady states of the system, and is the Hamiltonian used in most quantum-chemistry problems.  I've never really understood the relationship between the time-dependent and time-independent equations, especially since this is where all the "weirdness" of QM comes into play. 

--David Shobe, Ph.D., M.L.S.
S|d-Chemie, Inc.
phone (502) 634-7409
fax (502) 634-7724

Don't bother flaming me: I'm behind a firewall.

-----Original Message-----
From: Shobe, David 
Sent: Monday, July 12, 2004 10:55 AM
To: CCL computational chemistry list (E-mail)
Subject: RE: Question on molecular structure and QM

Wouldn't the isomers simply be different solutions to the same non-B-O Hamiltonian, just as the various states of a molecule are different solutions?  (By "the various states" I mean the ones that within the B-O approx. are labeled vibrational, rotational, and electronic). 

--David Shobe, Ph.D., M.L.S.
S|d-Chemie, Inc.
phone (502) 634-7409
fax (502) 634-7724

Don't bother flaming me: I'm behind a firewall.

-----Original Message-----
From: Computational Chemistry List [mailto:chemistry-request)at(ccl.net]On
Behalf Of Eric Scerri
Sent: Friday, July 09, 2004 3:00 AM
To: ccl
Subject: CCL:Question on molecular structure and QM

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.

-- 

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

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