CCL:G: CASSCF problems

 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:
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 >
 > --
 >   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>
 >