CCL:G: Problem about calculation on the open-shell singlet in Gaussian.

Dear Ping,

I guess that what you are after is a scf solution in which two electrons occupies different orbitals but are antiferromagnetically coupled?

In Gaussian you can first try to set the spin multiplicity to singlet but using udft (or uhf) and the command guess=mix or guess=(mix,always).

If this doesnt work you can first do a SP calc (with the commands above) on the geometry of the corresponding triplet state, since this will probably be more similar to the geometry of the open shell singlet than to that of the closed shell singlet. (this might also work without the mix keyword) If you get an open shell singlet, which you will se in the pop analysis, you can then relax the geom by starting a geom opt in this state.

Note that this open shell singlet is not an eigenfunction of the S^2 operator (it is a broken symmetry state in the single determinant formalism) and that it is spin contaminated by the corresponding triplet state. If the energy difference between the open shell singlet and the triplet is large you should correct the energy of the open shell singlet by using the formalism derived by Noodleman (J. Chem. Phys. 1981, 74, 5737, or Adv. Inorg. Chem. 1992, 38, 423).

Good luck!

/Johannes

--
Adam Johannes Johansson
Ph.D., M.Sc.
Institut für Organische Chemie
RWTH Aachen University
Landoltweg 1
52056 Aachen
Germany
Cell phone:0708178487
E-mail:johjo76/a\gmail.com
Research web page:www.physto.se/~johjo

2009/7/10 Huo-Lei Peng huolei.peng166|-|gmail.com <owner-chemistry/a\ccl.net>

Thanks for your reply. I'd like to do calculation on the open-shell singlet (not a triplet). Open-shell(U) and restricted (R) calculations in Gaussian gave me same results. So in Gaussian, how to set input to make two electrons in two different orbitals but we still have a singlet state?

Thanks.

Peng

On Fri, Jul 10, 2009 at 11:14 AM, Jose Alberto Fernandes jafernandes{:}ua.pt <owner-chemistry**ccl.net> wrote:

Sent to CCL by: "Jose Alberto Fernandes" [jafernandes=-=http://ua.pt" target="_blank">ua.pt]
Hello

I don't know if I understood your problem. Lets try.
Please see the following excerpt this input:

# rb3lyp/3-21g guess=check

The "r" before "b3lyp" means that you want a close shell calculation. If you don't want electron pairing you have to put "ub3lyp" instead of "rb3lyp".

You should remember that a closed shell compound has always a spin=1, and in a open shell the spin is different from 1. In the case of nitrene (R-N ?) you can have R-N paired (spin=1) and unpaired (spin=3).

So in the case of pairing you shoul input:

------------------------
# rb3lyp/3-21g guess=check

comment

0 1

geometry

----------------------
If you want unpairing you should input:

---------------------
# ub3lyp/3-21g guess=check

comment

0 3

geometry

----------------------
If I haven't answered your question, please explain better.

Best regards

Jose

> "Huo-Lei Peng huolei.peng166(!)gmail.com" wrote:
>
> Sent to CCL by: Huo-Lei Peng [huolei.peng166]=[gmail.com]
> --001636c5a4fb6b291a046e527474
> Content-Type: text/plain; charset=ISO-8859-1
> Content-Transfer-Encoding: 7bit

>
> Hi, Dear All:
> I have a question about calculation on the open-shell singlet using
> Gaussian. For example, nitrene species may have an open-shell and closed
> shell singlets. In Gaussian, how to set the calculation to get open-shell or
> closed-shell specifically? Thanks.
>
> Peng
>

> --001636c5a4fb6b291a046e527474
> Content-Type: text/html; charset=ISO-8859-1
> Content-Transfer-Encoding: quoted-printable
>
> Hi, Dear All:<div><br></div><div>I have a question about calculation on the=
> open-shell singlet using Gaussian. For example, nitrene species may have a=
> n open-shell and closed shell singlets. In Gaussian, how to set the calcula=
> tion to get open-shell or closed-shell specifically? Thanks.</div>
> <div><br></div><div>Peng</div>
>
> --001636c5a4fb6b291a046e527474--
>
>

-= 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/