Re: CCL:G: Triplet transition states

On Fri, Jun 20, 2008 at 1:27 PM, Clark, Aurora auclark#wsu.edu <owner-chemistry()ccl.net> wrote:

Hi Gary;

I think it is important to be clear about the type of open-shell singlet state that you could obtain with gaussian using DFT. Using unrestricted
B3LYP it should be possible to obtain an <S^2> =3D 1 broken-symmetry<= br> broken-spin 'singlet' state that really is a representation of the = mixture
of the true multi-determinant singlet (that you would obtain from CASSCF an= d
would have <S^2> =3D 0) and the triplet state (with <S^2> =3D 2= ). Often these
broken-symmetry singlet states can yield very useful information about your=
reaction coordinate, and there are ways to correct the energy of the
broken-symmetry singlet state to approximate what the true S=3D0 singlet st= ate
energy should be (See a number of papers by E. R. Davidson, Yamaguchi, and<= br> others).

If you want to obtain the broken-symmetry singlet state in Gaussian, there<= br> are a number of tricks in addition to the guess=3Dmix option that I have fo= und
successful. The most reliable option I have found is to create a fort.7 fil= e
of the triplet orbitals (with the punch=3Dmo keyword and using unrestricted=
b3lyp so that you get the "lowest energy" alpha and beta set of M= Os),
determine which alpha orbitals are the ones without an occupied beta
complement (most likely the top two occupied alpha orbitals), copy the alph= a
set into the beta set so that the alpha and beta MO's are exactly ident= ical,
then in the beta set, move one of the one of orbital that you want to= have
the spin down electron into the occupied space. You want to make sure that<= br> when you do the broken-symmetry calculation that the singly occupied alpha<= br> orbital and singly ocuppied beta orbitals are not the same and that they have the electrons roughly localized where you think they should be in the<= br> molecule. Then you can read in these alpha and beta orbitals using the
guess=3Dcards keyword and have the orbitals added in the end of your input<= br> file. This is a rather bootstrap method to create a broken symmetry guess that should have an <S^2> =3D1 at the beginning of the SCF. However, = the
benefit is that you know that you are giving Gaussian the best possible
guess of orbitals for the broken-symmetry state, and not relying on Gaussia= n
to do it for you. Of course, there is no guarantee that the broken-symmetry=
solution will be maintained after the SCF cycle.

I suggest you read a bit about broken-symmetry singlet states and look at the singly occupied alpha and beta orbitals in those papers so that you hav= e
a firm idea about where you want your alpha and beta electrons to be
localized for your reaction TS. Getting these broken-symmetry states can be=
quite tricky, however they will likely be faster than going to CASSCF
depending on the size of your active space.

I hope this helps;

Aurora

Aurora Clark
Assistant Professor
Department of Chemistry
Washington State University
Pullman, Wa 99164
Ph (509)-335-3362
Fax (509)-335-8867
Email: auclark]_[wsu.edu

On 6/20/08 7:51 AM, "Breton, Gary gbreton__berry.edu"
<owner-chemistry]_[ccl.net<= /a>> wrote:

>
> Sent to CCL by: "Breton, Gary" [gbreton|,|berry.edu]
> Hi everyone,
>
> I hope everyone is enjoying their summer. I have been working on a lon= g term
> project (possibly TOO long) in an attempt to locate possible transitio= n
> states for the thermal decomposition of a series of small ring heteroc= yclic
> compounds. Upon thermolysis, the compounds liberates nitric oxid= e (NO)
> which is a radical species (which could be important..see below).
>
> I started this project employing the DFT method at the B3LYP/6-311+G(d= ,p)
> level and located a number of transition states. Being a bit&nbs= p; na=EFve at the
> time, I had set the transition state calculation to search for a saddl= e
> point using the unrestricted B3LYP functional and at a triplet state.&= nbsp; I was
> able to reproduce these same calculations using MP2/6-31G(d) with few<= br> > additional optimization steps. IRC calculations were beautiful a= nd linked
> my starting materials to products as expected. The transition st= ates
> exhibited expectation values <S*2> of about 2 as expected (since= I
> intentionally searched for triplet states).
>
> Later on, after reviewing a number of papers, I began to realize what = I
> needed to do was search for the same TS using unrestricted (open-shell= )
> procedures but optimizing to a singlet state.   Under these = conditions, I
> cannot locate ANY of the transitions states even though I have tried > manipulating the starting geometries of the input TS structures as wel= l as
> through use of the QST2 search feature in Gaussian. I have emplo= yed the
> Guess=3D Mix option in these calculations as well. The calculati= ons fail
> after the molecule essentially blows apart or relaxes to something rel= ated
> to the starting material with a residual imaginary frequency.
>
> While I am beginning to come around to the fact that CASSCF calculatio= ns or
> something similar may be needed here (which I admit to being unqualifi= ed to
> carry out), is there something salvageable from all these calculations= ? Is
> there a situation where a triplet optimized TS means something? = Or is there
> some "trick" to getting these TS structures (or similar) to = find to a saddle
> point as singlet structures?
>
> I hope I've provided enough information for some answers. Th= anks for any
> suggestions.
>
> Best regards,
>
>
> Gary W. Breton
> Chair and Prof. of Chemistry
> Berry College
> PO Box 495016
> Mount Berry, GA 30149
>
>
>
>
>
>
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