Sent to CCL by: Thomas Exner [thomas.exner*|*uni-konstanz.de] Dear Jürgen:Thank you very much for pointing me to the FON-DFT approach. I will definitely look into it. But I still think that standard CASSCF should be able to get it right. Perhaps I make a complete fool out of me, anyway: In an MO picture, there are the two degenerated states (pi_x and pi_y) and HF and DFT, which take no non-dynamic correlation into account, should convert to one of these. But the NO molecule should show a symmetric electron distribution, which shows, in my opinion, that the MO scheme is misleading. Fractional occupation numbers help here, since one can put half an electron in each of the two degenerated pi orbitals. But is that not exactly what CASSCF is meant for. By combining multiple electron configurations, the calculation should be able (in a very simplified way to demonstrate my point) to take the two configuration with the unpaired electron in the pi_x orbital and in the pi_y orbital, respectively, with equal contribution. This would then result in an electron occupation of 0.5 in both orbitals, exactly what I expect. Any comments on that or a reason, why I am wrong?
And, really no suggestions for my second question?"Additionally, I have a larger system for which I also perform casscf calculations. Ground state simulations and also the first excited state using the ground state geometry are fine. Energy optimization in the excited state also starts ok but after some steps the calculation does not converge anymore. Use of "use=l506" as proposed in the g09 manual did not help. Also no luck with increasing the maxcycle or starting with an other conformation. Is there anything else I can try? Perhaps there are some options for the optimizer that he does not jump into the bad region with the convergence problems. Unfortunately, the "sleazy" and the "qc" keyword for scf cannot be used with casscf optimization."
Thanks. Thomas Jürgen Gräfenstein jurgen**chem.gu.se wrote:
Sent to CCL by: =?iso-8859-1?Q?J=FCrgen_Gr=E4fenstein?= [jurgen:_:chem.gu.se] Dear Thomas, Everything appears to be all right. The state you seek is twofold degenerate (Pi_x, Pi_y), and a quantum chemical calculation gets you one of the states. In order to get an equal distribution between Pi_x and Pi_y youwould need a fractional-occupation-number (FON) CASSCF. I am not sure whether something like this has been worked out and implemented somewhere for CASSCF. (FON-DFT is implemented in Gaussian). Best regards, Jürgen ---- Jürgen Gräfenstein University of Gothenburg Dept of Chemistry Jurgen.Grafenstein!A!chem.gu.se On 10 Dec, 2011, at 15:09 , Thomas Exner thomas.exner%a%uni-konstanz.dewrote:Sent to CCL by: Thomas Exner [thomas.exner%%uni-konstanz.de] Dear CCLers: I am running in some problem with CASSCF calculations using g09, for which I have no explanation, and I hope that somebody out there can help me. I would like to start with a relative simple system: NO. For this system I would expect that the unpaired electron is distributed equally between the two antibonding pi orbitals. But I get an occupation number of exactly 1 for the first and 0 for the second orbital using a casscf(1,2) calculation. In a casscf(5,4) including the two bonding pi orbitals, there is again almost exactly 1 electron in the one orbital and a few percent ofan electron in the other. The additional electron occupation is taken from the bonding orbitals. Has somebody an idea, what I am going wrong, or is my assumption of equal distribution wrong? Here is the input (pretty simplex, eh): # casscf(5,4)/sto-3g nosymm NO 1 0 2 O N 1 B1 B1 1.25247685>
-- ________________________________________________________________________________ Dr. Thomas E. Exner Fachbereich Chemie Universität Konstanz 78457 Konstanz Tel.: +49-(0)7531-882015 Fax: +49-(0)7531-883587 Email: thomas.exner .. uni-konstanz.de ________________________________________________________________________________