From owner-chemistry@ccl.net Tue Dec 13 16:44:00 2011 From: "Kirk Peterson kipeters ~ wsu.edu" To: CCL Subject: CCL:G: CASSCF problems Message-Id: <-46029-111213164242-1870-nM9HDi7z4eLaYBu6Rd7UMw!^!server.ccl.net> X-Original-From: Kirk Peterson Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=iso-8859-1 Date: Tue, 13 Dec 2011 13:42:33 -0800 Mime-Version: 1.0 (Apple Message framework v1251.1) Sent to CCL by: Kirk Peterson [kipeters+/-wsu.edu] Dear Thomas, the calculation that Brian suggests below is easily done with Molpro by carrying out a state-averaged CASSCF or HF calculation. In this case the Pi_x and Pi_y states are computed with equal weights and the final natural orbital occupation numbers will show an occupancy of 0.5 for each of the open-shell pi orbitals. I always use this type of symmetry equivalencing for degenerate states of linear molecules (Pi, Delta, etc.) and particularly for open-shell atoms. regards, Kirk On Dec 13, 2011, at 1:01 PM, Brian Salter-Duke brian.james.duke * gmail.com wrote: > > Sent to CCL by: Brian Salter-Duke [brian.james.duke]^[gmail.com] > On Tue, Dec 13, 2011 at 07:14:20PM +0100, Thomas Exner thomas.exner . uni-konstanz.de wrote: >> >> 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? > > I think the answer is that either HF solution gives the same energy as > txpeh mix. However, it used to be possable back in the 1970 in the ATMOL > program in UK to run what was called the symmetry equivalenced > Hartree-Fock method (SEHRF) which gives equal weight to the odd electron > being in pi-x and pi-y. See M F Guest and V R Saunders, Mol Phys., 28, > 819, (1974). I do not know whether it is in any other code. It might be > in GAMESS(UK) which is still developed by Martin Guest. > > Brian. > >> 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 >> ________________________________________________________________________________http://www.ccl.net/chemistry/sub_unsub.shtmlConferences: >> http://server.ccl.net/chemistry/announcements/conferences/> > > -- > Brian Salter-Duke (Brian Duke) Brian.Salter-Duke(a)monash.edu > Adjunct Associate Professor > Monash Institute of Pharmaceutical Sciences > Monash University Parkville Campus, VIC 3052, Australia> >