Re: CCL:Theta_0 values for polyatomics



Hi,
 > 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?
 The differences between equilibrium structures as calculated by computational
 chemistry methods and "measured" structures are well understood and
 there
 is a strict theoretical framework for calculating them, although it is very
 expensive (you need third derivatives) and therefore not very widely done=2E
 As a starting point you might want to check out:
 J.-R. Hill, J. Sauer, and R. Ahlrichs
 Ab Initio Calculation of Nuclear Motion Corrections to the Geometries of
 Water, Methanol and Silanol
 Mol. Phys. 73(2) (1991), 335 - 348
 and references therein. The entire theoretical derivation can be found in:
 M. Toyama and T. Oka and Y. Morino
 Effect of vibration and rotation on the internuclear distance
 J. Mol. Spectrosc. 13 (1964), 193
 Hope this helps
 J=F6rg-R=FCdiger Hill
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