Subject: summerize: IRC question

From: "dwang" <[email-masked]>
Date: Tue, 12 Jun 2001 23:58:41 +0800

Dear cclers,

  The following is the replies to my question which was posted a few weeks ago. Thanks a lot to Dr. Matthias Hofmann and Dr.Douglas J. Fox.

>>>>>>>>>>>original question<<<<<<<<<<<<<<<< 
> 
> Dear Sir/Madam
>   I am performing a IRC program, A->TS->D. By using opt=3Dqst2 , I found =
> the structure of TS. But the irc(result B ->C) didn't result in what I =
> want. So I think the path may be A ->TS1 ->B ->TS0 -> C -> TS2 ->D. I =
> repeat the calculation by using C and D as input file to find TS2, then =
> IRC. While the results indicate that this IRC included the IRC I have =
> performed first. Why? Any advice will be highly appreciated.
>>>>>>>>>>>>>>>>>>>>>>>>><<<<<<<<<<<<<<<<<<<<<<<<<<<<<
Hello, 
Regarding your IRC problems: 
You were not very specific how you did your calculations. 
I recommend, to use internal coordinates (Z-Matrix) for the 
IRC runs and to separate into a forward and a reverse job. 
Sometimes the IRC stops very early and reports it has reached 
a minimum while  this is not true. (Just run a opt for the structure
to confirm it as a minimum) 
When trying to restart an IRC the output becomes completely mixed
up unless you restrict the run to one direction. 

Good luck, 
Matthias Hofmann
----------------------------------------------------------------------------
  Dr. Matthias Hofmann
  Anorganisch-Chemisches Institut               
  Ruprecht-Karls Universitaet Heidelberg           Tel.: (INT+) 6221 54-8451
  In Neuenheimer Feld 270                          Fax.: (INT+) 6221 54-8439
  D-69120 Heidelberg          e-mail: [email-masked]
  Germany            http://www.uni-heidelberg.de/institute/fak12/AC/hofmann  
----------------------------------------------------------------------------

Dr. Wang,
  Let me restate this my way just to make sure I understand.

  For this simple system you are mapping out the full potential energy
surface including isomers for Cl attacking at C and S, the transition
state for isomerization and possibly some other decomposition products.  At
this point you have found a few local minima and a few transition
state candidates.  Is this accurate?

   Assuming it is I have a couple of questions.  First, are you monitoring
the spin state here?  For the Cs structures there is at least a doublet
A' and A" state.  Probably only two relevant states for the C2v structure.
If you are not staying on one of these surfaces then I could see some
of the complexity here relating to having two surfaces.  If all of these
structures are not planar then you would have only one doublet state but
it will connect to only one of the Cs/C2v surfaces. So watch that you are
not mixing states.

   Second, DFT is notorious for underestimating barrier heights.  So
once you have identified the principle paths you should look at how
MP2 or some traditional correlation method affects this.  Since you have
IRC paths you might want to use IRCMax to cut down on the effort.
It will let you trace MP2 energies along this path and get close to
the MP2 TS structures.  Then you can reoptimize the ones which show
promise.

   Third, are you checking the FREQ results at each of the TS structures
to verify that only one imaginary mode is present?  Also that the 
barrier heights are relevant wrt the the zero point energy if these
are rather shallow maxima/minima.  If you are having problems getting
the optimization tight enough with DFT consider improving the integration
grid, INT=GRID=UltraFine for example, to make sure that the optimizer
is not being confused by numerical error.

   Finally, have you compared your surface with the CO2 analog?  Certainly
the chemistry will be different but I would expect it to have a lot of
similar features. 

  Douglas J. Fox
  Technical Support
  Gaussian, Inc.
  [email-masked]

 >>>>>>>>>>>>>>>>>>>>My letter to Dr. Fox<<<<<<<<<<<<<<<<<<<<<<<<<< 
> Dear Dr. Fox,
>   Thank you for your help.
>   Now  I am performing the reformation reaction between the carbon isomer(A)
> and sulfur isomer(B) of CS2Cl radical(B ->TS ->A)(method: DFT, basis set:
> 6-311G**)(the radicals come from CS2+chlorine atom radical). I have supposed
> the sulfur isomer(symmetry: Cs) is more unstable than the carbon
> isomer(Symmetry: C2v), so A will be the reactant. I have found the transient
> state by using qst2 and two intermediates(C and D)by performing IRC. Then I
> finished the optimization of C and D(result in C1 and D1 respectively), and
> found that the structure of C1 is almost the same with A and D1 is the same
> with B(So the path may be A ->TS ->B?).
>   I think maybe D can transform into D1 through path D ->TS2 ->D1, so I
> performed TS2-searching by using qst2( between D and D1), then IRC(end points: E and F). The result indicated that this IRC included almost the
> whole path , and E is almost the same with C1. To F and D1, the bond
> lengthes are almost the same , while there is a rather large energe gap
> (about 0.010 hatree)between F and D1, and the bond angle of (C-S-Cl) of F is much less than D1( about 71.2 and 105.2 respectively).
>   Now I have found another transient state between F and D1 still by qst2
> and the IRC is in progress. This time I added the keyword Opt=(verytight,
> reverse) to the path. I don't know if this is reasonable because I have
> little experience in IRC calculation.
> 
>   This is a system only including four atoms and I have some bigger systems
> waiting for calculation. So I wish I can learn much from the IRC of CS2Cl in
> order to perform the coming work well. Could you give me some advice?
>   Thank you very much!
>>>>>>>>>>>>>>>>>>>><<<<<<<<<<<<<<<<<<<<<<<<<<<<<

Dr. Wang,

   The use of QST2 improves the chances that you will zero in on the
transition state of interest however, there is always the possibility
that either your expectations are wrong or the model chemistry you are
using does not match reality well.

   So what I read here you have found a transition state which does
not connect to either the product or reactants you expected.  While
your three-part reaction mechanism is possible it seems a bit involved
if you actually see the results of this reaction.  If this four part
reaction sequence should also result in measurable amounts of these other 
products.  Are they seen or at least plausible?

   I don't quite understand your last comment except it reminds me that
perhaps you are confused that the IRC attempts to produce the actual
optimized reactant/product structures as the end points.  Rather it does
a fixed number of steps and from that you can generally conclude that
it is in the right product/reactant valley.  If you then looked for the
path from C to D it would coincide with the earlier path.

   Perhaps if you describe this more completely I could make a better 
determination of what is up.  What methods are you using?  What are the
structures A,B,C,D?

  Douglas J. Fox
  Director of Technical Support
  [email-masked]