Subject: photoreaction with g03?

From: Rupert Mazzucco <maz*at*ap.univie.ac.at>
Date: Mon, 4 Aug 2003 19:56:12 +0200 (CEST)

Hi,

in an attempt to better understand an optical absorption spectrum
we have measured, I'd like to simulate the photodecay of DMPA
(2,2-dimethoxy-2-phenylacetophenone, C16O3H16, common photoinitiator
for free radical polymerisation) with Gaussian 03.  I have no
prior experience with Gaussian, other than playing around with it
for two weeks and trying some of the examples in this introductory
book that comes with it (and I'm not even chemist, but physicist),
so I'd be glad for any help.

A paper I've found explains the reaction as follows:  The molecule
in its (singlet) ground state absorbs a photon and goes into an
excited singlet state, which decays into a triplet state by "intersystem
crossing"; then the C-C bond in the middle of the molecule breaks and
leaves two radicals.  (Then there is a photoinduced follow-up reaction,
but that one is for later.)  I want to calculate all the states that
occur in that reaction.

So (after an initial geometry optimisation, tried different methods
which give nearly identical results) I'm using the RCIS method to get
singlet and triplet excited states, restricted shell because the
ground state is a singlet (right?).  I've been advised to use a **
basis set because of the oxygens, and the manual says to use diffuse
base functions, so I'm going with 6-31+G(p,d) for now.  Then  I'll try
to locate an excited singlet state that corresponds roughly to the photon
energy as well as the triplet state below it, and do a OPT FREQ on them.

Does that sound like a good plan so far?  There is also the TD method,
which seems to serve a similar purpose.  Maybe I'll try that later.

The manual says that excited state calculations are not very accurate.
What accuracy can I expect, how far will energies be off, roughly?

Thanks for any input.

Best regards,
Rupert Mazzucco
maz*at*ap.univie.ac.at