Subject: Question about semi-empirical vs. tight-binding methods for a (100 atom) molecule

From: Connie Chang <[email-masked]>
Date: Thu, 10 Oct 2002 00:21:30 -0400

Hi --

I am trying to find the vibrational modes of a 100 atom molecule that
includes elements such as carbon, nitrogen, hydrogen, and cobalt (just
one atom).  I tried using commercial software such as Gaussian and
Hyperchem, but was having trouble getting "sensible" answers with their
semi-empirical methods and ab-initio methods proved to be too
computer-intensive.  

I'm trying to write my own code to calculate energies (so that I can
construct a hessian for the phonon spectra), but I'm not sure about the
methods out there.  Is tight-binding the same as the semi-empirical
methods?  I know that in tight binding, one needs to find the parameters
for the elements in the molecule and the parameters for the particular
atomic connectivity (so if my molecule has a carbon bonded to an oxygen,
I would need the parameters for a carbon-oxygen connectivity).  Is this
how semi-empirical methods work as well?  Or is there just a set of
parameters for each element without taking into account the
connectivity?  Or is there some systematic way to derive parameters
between elements?

Any advice about what method to use or resources that I can look at
would be much appreciated.

-Connie Chang