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

*From*: "Jens Spanget-Larsen"
<spanget*at*virgil.ruc.dk>
*Organization*: Roskilde Universitetscenter
*Subject*: Re: CCL:Summary: Calculations without Born-Oppenheimer
approximation "2nd Call"
*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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