Subject: Re: bsse and second derivatives in G98

From: "Artem Masunov" <[email-masked]>
Date: Fri, 28 Jan 2000 13:05:08 PST

>Herbert Fruchtl writes:
>Well, the derivative of nothing isn't actually zero - but nothing.
>In the BOA it simply doesn't make sense to talk about _that_ Hessian.

This discussion is getting a bit metaphysical..

>The Argon can't feel a force,
>because in this approximation forces only exist between nuclei. The

First note that MO LCAO approximation is not a part of BOA.
In the perfect world with complete basis sets (or numerical
wavefunctions) you would be right. But we have to deal with atom
centered basis functions, which don't even satisfy Hellman-Feinman
theorem.. And we (OK, some of us) are just trying to make the results
a little better.

Now, when you move your basis function and leave the atomic nuclei
in place, does the energy change? Yes. Can we look at the derivatives
with respect to this displacement? Sure. Does it have something to do with 
physical frequency? As a term in correction formula. Is it part
of Counterpoise correction? If it's not, we'll just have to adjust the 
definitions.

>as I understand Counterpoise, it can only be used to calculate wavefunction 
>properties (e.g. the energy, but not the Hessian).

OK, a derivative of a sum equals the sum of the derivative, doesn't it? So 
to get Hessian of the corrected energy you'll need Hessians of all the 
corrections...

And if not all the corrections have physical meaning, blame your bais for 
that.

Artem
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