Subject: Gaussian03: MP2 before CI or CC

From: Nicolas Ferré <nicolas.ferre(at)up.univ-mrs.fr>
Date: Tue, 14 Dec 2004 16:14:47 +0100

Dear all,

When you run a CI or CC job with g03, it starts by resolving the HF 
equations. In the next step, the integrals are expressed over MOs and 
then the first-order MP wavefunction (and the second-order MP energy) 
are computed, all of this done in the link 804. Finally, g03 uses these 
informations for the CI or CC job in the link 913.
I would like to find references about this procedure. I guess it is 
related to the Davidson scheme for diagonalizing the CI matrix. But why 
always using the MP wavefunction instead of the HF one ? Is there an 
option in g03 for using the HF wavefunction ?

What is more surprising is that the CI or CC energy depends on the MO 
eigenvalues ! I just slightly modified the code in link 801 for changing 
one eigenvalue (rwf 547) and the final CI or CC energy varies with this 
eigenvalue ! I checked the same behavior on different machines. Can 
someone explain what is happening ? I guess I am missing something but I 
cannot find what.

-- 
  Dr. Nicolas Ferre'
  Laboratoire de Chimie Theorique et de Modelisation Moleculaire
  UMR 6517 - CNRS Universite' de Provence
  Case 521 - Faculte' de Saint-Jerome
  Av. Esc. Normandie Niemen
  13397 MARSEILLE Cedex 20 (FRANCE)
  Tel : (+33)4.91.28.27.33              Fax : (+33)4.91.28.87.58

  Please avoid sending me Word or PowerPoint attachments.
  See http://www.gnu.org/philosophy/no-word-attachments.html