Subject: Theta_0 values for polyatomics

From: John Kerkines <[email-masked]>
Date: Fri, 23 Jun 2000 11:34:16 +0300 (EET DST)

Dear listmembers,

In the simple case of a diatomic molecule, we know that the internuclear
distance re (well depth) and r0 (zero-point corrected) differ slightly due
to the anharmonicity of the potential function. That's why r0 > re. But,
you can 'estimate' the difference r0 - re fairly easily. (One can easily
see the difference by just looking at the energy vs distance plot).

In the most complex case of polyatomics, you have 3N-5 or 3N-6 degrees of
freedom. For the simpler case of a bent triatomic molecule as for example
H2O, you have a Theta_e angle that you calculate by minimizing the energy,
but an experimentalist should be measuring a Theta_0 value. Most of the
time, calculations compare their theta_e results with an experimental
theta number, whatever its subscript. I know the difference between
theta_e and theta_0 should be very small in most triatomics, but sometimes
an experiment may report both values, their difference being as much as 1
degree. 

I want to find out how I can 'correct' a theta_e value I calculate in some
level of theory, to theta_0, by incorporating in some way (which one?) the
zero-point contributions.

As one can see, this is a special case in a big problem which will be more
and more evident in the future, as calculations become more and more
accurate. To what degree of accuracy are experimental and theoretical
results comparable, since 'theory decides what is measurable', but on the 
other hand we don't really measure EXACTLY the same thing?

Regards,

Ioannis S.K. Kerkines, MSc
PhD Candidate in Physical Chemistry
Department of Chemistry
University of Athens, Greece
Panepistimiopolis Zografou, 157 10
Tel: +301-727-4569, Fax: +301-723-3219
Email: [email-masked]