Subject: Re: CCL:Summary: Calculations without Born-Oppenheimer approximation "2nd Call"

From: "Jens Spanget-Larsen" <spanget*at*virgil.ruc.dk>
Date: Thu, 8 Jan 2004 11:40:50 +0100

"John McKelvey" <jmmckel*at*attglobal.net>:

> Jens,
> 
> A question, please.  Neglecting the fact that FSGO's may be poor basis
> functions, where do they stand with respect to BO if the positions of the
> gaussians are optimized along with the SCF process?
> 
> Regards,
> 
> John McKelvey
> 

Dear John: 

I don't have the papers at hand. But as far as I see, the 
basic features of the approach by Boehm and associates are the 
following: They perform the basic calculations within the 
usual Born-Oppenheimer approximation, obtaining data for a 
very large number of nuclear configurations. This part of the 
investigation can be performed with a variety of electronic 
hamiltonians and basis sets, as in other standard ab initio 
procedures. However, and that is the important point, they 
don't determine the molecular "equilibrium configuration" in 
the usual Born-Oppenheimer manner, i.e., by selecting the 
single nuclear configuration that corresponds to a minimum on 
the Born-Oppenheimer potential hypersurface. In the Boehm 
procedure, the quantum and thermal nuclear degrees of freedom 
are accounted for by forming quantum ensemble averages over 
the set of nuclear configurations. The result is a wave-packet 
picture of the nuclear positions that goes beyond the Born-
Oppenheimer picture.

Yours, Jens >--<

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JENS SPANGET-LARSEN         Office:         +45 4674 2710
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