CCL:G: Gaussian question

Dear Tijesunimi,

First of all, MP2 introduces electron correlation via Rayleigh-Schroedinger perturbation theory using occupied and virtual orbitals and their energies from Hartree-Fock calculation, so it does not create new orbitals or orbital energies. Therefore FMOs > from your MP2 calculation are de facto HF orbitals. 
Second, it is well known that DFT gives different orbital energies than HF (but you cannot say they're worse). For detailed discussion you can check e.g.: 

Stowasser, R., & Hoffmann, R. (1999). What Do the Kohn−Sham Orbitals and Eigenvalues Mean? Journal of the American Chemical Society, 121(14), 3414-3420.

Kar, T., Ágyá J. G., & Sannigrahi, A. B. (2000). Comparison of ab Initio Hartree−Fock and Kohn−Sham Orbitals in the Calculation of Atomic Charge, Bond Index, and Valence. The Journal of Physical Chemistry A, 105(3), 9953-9963.

Zhang, G., & Musgrave, C. B. (2007). Comparison of DFT methods for molecular orbital eigenvalue calculations. The Journal of Physical Chemistry A,111(8), 1554-1561.

Best regards,
Piotr Nowak

On Tue, Jan 25, 2011 at 8:42 PM, Tijesunimi John Odebode tijesunimi=yahoo.com <owner-chemistry*|*ccl.net> wrote:

Sent to CCL by: "Tijesunimi John Odebode" [tijesunimi:-:yahoo.com]
Hello all,

I was wondering if I could get an answer to this question:

I did a geometry optimization of a compound using Gaussian(b3lyp/6-31G(d)). I found the HOMO and LUMO energy and calculated the energy gap. I then decided to do an MP2 single point calculation on the same compound to determine the HOMO and LUMO energy gap. The energy gap for the single point MP2 calculation was 3.49eV  more than that for the b3lyp. Is this normal? Or do you think there is a problem somewhere? Has anybody carried out such calculations? Thanks a lot for your anticipated response.



-= This is automatically added to each message by the mailing script =-
E-mail to subscribers: CHEMISTRY*|*ccl.net or use:
     http://www.ccl.net/cgi-bin/ccl/send_ccl_message

E-mail to administrators: CHEMISTRY-REQUEST*|*ccl.net or use
     http://www.ccl.net/cgi-bin/ccl/send_ccl_message

Subscribe/Unsubscribe:
     http://www.ccl.net/chemistry/sub_unsub.shtml

Before posting, check wait time at: http://www.ccl.net

Job: http://www.ccl.net/jobs
Conferences: http://server.ccl.net/chemistry/announcements/conferences/

Search Messages: http://www.ccl.net/chemistry/searchccl/index.shtml
     http://www.ccl.net/spammers.txt

RTFI: http://www.ccl.net/chemistry/aboutccl/instructions/





--
******
Piotr Nowak
University of Warsaw
Department of Chemistry
Laboratory of Stereocontrolled Organic Synthesis
Pasteura 1, 02-093 Warsaw, Poland
******