From owner-chemistry@ccl.net Wed May 16 03:36:00 2007
From: "Ulrike Salzner salzner++fen.bilkent.edu.tr" <owner-chemistry[-]server.ccl.net>
To: CCL
Subject: CCL:G: CASMP2 (G03)
Message-Id: <-34274-070515112739-10757-yyiaQa/TLlivRPq0zoEFUw[-]server.ccl.net>
X-Original-From: Ulrike Salzner <salzner(_)fen.bilkent.edu.tr>
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Date: Tue, 15 May 2007 17:41:27 +0300
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Sent to CCL by: Ulrike Salzner [salzner^-^fen.bilkent.edu.tr]

Hello,
while trying to calculate the excitition energies of the bithiophene 
dication with CASMP2 using Gaussian 03 I ran into the following problem.
I am using all of the pi-electrons and pi-orbitals (10 electrons and 10 
orbitals). I got the ground and 5 excited states at the CASSCF level 
without problems. Then I tried CASMP2 with the same active space. First 
there is a warning that there are more than 10000 configurations, there 
are actually about 63504. In the MP2 part all confuguratons are skipped 
and then it prints:
"Multireference MP2 with 0 configurations"
and the energy is same as without MP2. The same happens for ground and 
first excited state. What is the problem? Is the active space too large? 
After reducing the active space to (8,8), MP2 runs using 21 configurations.
How are the configurations selected? How reliable is such a result? Will 
it use more configurations, if I reduce the active space further?
Thanks for assisstance,
Ulrike