Subject: Question about semi-empirical vs. tight-binding methods

From: Connie Chang <[email-masked]>
Date: Thu, 10 Oct 2002 11:50:09 -0400

Hi --

I'm trying to find the vibrational modes of a molecule (~100 atoms)
which has the elements carbon, nitrogen, oxygen, and cobalt in it.  I've
tried doing this through commercial programs such as Hyperchem and
Gaussian, using the semi-empirical methods they provide (since the
ab-initio methods are too computer-intensive); but I'm not getting
sensible results.  I suspect that part of the problem is because of the
presence of the cobalt.

My advisor wants me to write my own code now with the logic that code we
write would be completely under our control as to what variables we
tweak...  My question is which method would be the best to do this.

Are semi-empirical methods the same as tight-binding methods?  I know
that in tight binding methods, you use parameters for the Hamiltonian
matrix elements.  And that you need to find the specific parameters for
the atom connectivity in your particular case.  For example, since I
have a carbon bonded to a nitrogen for my molecule, I need to find
parameters specific to carbon-nitrogen connectivity.  My question is is
there a way in either the semi-empirical or tight binding methods where
i would just need the parameters of the individual elements and there
would be some systematic way for me to get the parameters *between*
elements?

Any suggestions, resources, etc. would be very helpful and appreciated.

Thanks,
Connie Chang
[email-masked]

---

From: Connie Chang <[email-masked]>
Date: Thu, 10 Oct 2002 14:10:02 -0400

Hi Thomas --

Thanks for the reply.  I'm still really learning about all of this...  Do you
know if tight-binding (which physicists use) and semi-empirical methods like PM3,
CNDO, etc. (which chemists use) are in fact the same?  Also, is it sufficient to
get parameters for the individual elements in the molecule or are parameters
describing an element's connection to another element needed?

Again, thanks.

Connie

Thomas Heine wrote:

> Hi,
>
> I can provide a code and CHN parameters, but Co is not parameterised afaik.
>
> You might ask Marcus Elstner ([email-masked]), he is
> working on biological systems using a TB method.
>
> Good luck
> Thomas
>
> Connie Chang schrieb:
> > Hi --
> >
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