From owner-chemistry@ccl.net Fri Jun 20 18:30:01 2008 From: "John McKelvey jmmckel|a|gmail.com" To: CCL Subject: CCL:G: Triplet transition states Message-Id: <-37220-080620164341-19737-8U3IgHviOxVryMVksAJS5A.:.server.ccl.net> X-Original-From: "John McKelvey" Content-Type: multipart/alternative; boundary="----=_Part_18481_33427087.1213994247817" Date: Fri, 20 Jun 2008 16:37:27 -0400 MIME-Version: 1.0 Sent to CCL by: "John McKelvey" [jmmckel*|*gmail.com] ------=_Part_18481_33427087.1213994247817 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline I'm not sure where the following fits related to the issue of triplet instabilities of singlet systems being discussed here, but I looked at this issue in 1972 using G70 to study the issue for ethylene. The pure singlet geometry was optimized using the 4-31 basis set. Using the 4-31 MOs for H2N-BH2 for the UHF alpha MO guess and those from H2B-NH2 for the beta MO's a sizable value of S**2 was obtained. Then I started decontracting the 4-3= 1 valence orbitals, and S**2 decreased significantly. Finally I decontracted the "core" and S**2 dropped a bit more, finally very close to the singlet value.. Sometime later someone followed up on the issue and pointed out that if the geometry was optimized along with making the basis set bigger that the UHF instability essentially went away. my $0.02.. Cheers, John McKelvey 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 =3D 1 broken-symmetry > broken-spin 'singlet' state that really is a representation of the mixtur= e > of the true multi-determinant singlet (that you would obtain from CASSCF > and > would have =3D 0) and the triplet state (with =3D 2). Often t= hese > broken-symmetry singlet states can yield very useful information about yo= ur > reaction coordinate, and there are ways to correct the energy of the > broken-symmetry singlet state to approximate what the true S=3D0 singlet > state > energy should be (See a number of papers by E. R. Davidson, Yamaguchi, an= d > others). > > If you want to obtain the broken-symmetry singlet state in Gaussian, ther= e > are a number of tricks in addition to the guess=3Dmix 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=3Dmo keyword and using unrestrict= ed > 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 ha= ve > the spin down electron into the occupied space. You want to make sure tha= t > when you do the broken-symmetry calculation that the singly occupied alph= a > 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 th= e > 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 inpu= t > file. This is a rather bootstrap method to create a broken symmetry guess > that should have an =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 > Gaussian > to do it for you. Of course, there is no guarantee that the broken-symmet= ry > 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" > 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 na=EFve = 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 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 th= e > > Guess=3D Mix option in these calculations as well. The calculations fa= il > > 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 calculation= s > or > > something similar may be needed here (which I admit to being unqualifie= d > 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 a= ny > > suggestions. > > > > Best regards, > > > > > > Gary W. Breton > > Chair and Prof. of Chemistry > > Berry College > > PO Box 495016 > > Mount Berry, GA 30149 > > > > > > > > > > > > > > -=3D This is automatically added to each message by the mailing script = =3D- > > To recover the email address of the author of the message, please chang= e > > the strange characters on the top line to the ]_[ sign. You can also> > > > Search Messages: http://www.ccl.net/htdig (login: ccl, Password: > search)> > > > > > ------=_Part_18481_33427087.1213994247817 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline I'm not sure where the following fits related to the issue of triplet i= nstabilities of singlet systems being discussed here, but I looked at this = issue in 1972 using G70 to study the issue for ethylene.  The pure sin= glet geometry was optimized using the 4-31 basis set.  Using the 4-31 = MOs for H2N-BH2 for the UHF alpha MO guess and those from H2B-NH2 for the b= eta MO's a sizable value of S**2 was obtained.  Then I started dec= ontracting the 4-31 valence orbitals, and S**2 decreased significantly.&nbs= p; Finally I decontracted the "core" and S**2 dropped a bit more,= finally very close to the singlet value..  Sometime later someone fol= lowed up on the issue and pointed out that if the geometry was optimized al= ong with making the basis set bigger that the UHF instability essentially w= ent away.

my $0.02..

Cheers,

John McKelvey

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
>
>
> 
>
>
>
> -=3D This is automatically added to each message by the mailing script= =3D-
> To recover the email address of the author of the message, please chan= ge
> the strange characters on the top line to the ]_[ sign. You can also >
>
>
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