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> = 1 broken-symmetry
broken-spin 'singlet' state that really is a representation of the mixture
of the true multi-determinant singlet (that you would obtain from CASSCF and
would have <S^2> = 0) and the triplet state (with <S^2> = 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=0 singlet state
energy should be (See a number of papers by E. R. Davidson, Yamaguchi, and
others).
If you want to obtain the broken-symmetry singlet state in Gaussian, there
are a number of tricks in addition to the guess=mix option that I have found
successful. The most reliable option I have found is to create a fort.7 file
of the triplet orbitals (with the punch=mo keyword and using unrestricted
b3lyp so that you get the "lowest energy" alpha and beta set of MOs),
determine which alpha orbitals are the ones without an occupied beta
complement (most likely the top two occupied alpha orbitals), copy the alpha
set into the beta set so that the alpha and beta MO's are exactly identical,
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
when you do the broken-symmetry calculation that the singly occupied alpha
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
molecule. Then you can read in these alpha and beta orbitals using the
guess=cards keyword and have the orbitals added in the end of your input
file. This is a rather bootstrap method to create a broken symmetry guess
that should have an <S^2> =1 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 Gaussian
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 have
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> 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 long term
> project (possibly TOO long) in an attempt to locate possible transition
> states for the thermal decomposition of a series of small ring heterocyclic
> compounds. Upon thermolysis, the compounds liberates nitric oxide (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 naïve at the
> time, I had set the transition state calculation to search for a saddle
> point using the unrestricted B3LYP functional and at a triplet state. I was
> able to reproduce these same calculations using MP2/6-31G(d) with few
> additional optimization steps. IRC calculations were beautiful and linked
> my starting materials to products as expected. The transition states
> 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 well as
> through use of the QST2 search feature in Gaussian. I have employed the
> Guess= Mix option in these calculations as well. The calculations fail
> after the molecule essentially blows apart or relaxes to something related
> to the starting material with a residual imaginary frequency.
>
> While I am beginning to come around to the fact that CASSCF calculations or
> something similar may be needed here (which I admit to being unqualified 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. Thanks 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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