Dear Keshav

One approach you may take is considering that spin reorganization (that is intersystem crossing from triplet to singlet) takes place after the transition state, in this case you need the transition state as a triplet as your global reaction will be:

3Formaldehyde → 3TS → 1[CO + H2]

You can try to find the 3TS using two paths:

1) Optimize both formaldehyde and the CO + H2 complex in the triplet state, and then use the QST2 algorithm considering that all structures are triplets (if QST2 fails you may try to use relaxed scans)

2) Optimize by any method the TS in the singlet state and after completion reoptimize it in the triplet state. This method only works if the 3TS structure is similar to the 1TS one, which may not.

Hope you find it helpful

thank you Enrique Manuel Arpa for your informative answer. i will try to search TS with your suggestions and ask again if i face any problem.

Dear Keshav,

The problem is that reagent and products belong to different Potential energy surfaces (PES), due to the fact that spin multiplicities are different.

Because of that, in this particular case, you can not find a transition state with usual methods as QST2, QST3.

You can follow what Enrique suggested: reagents and products have the same multiplicity of spin—> they belong to the same PES, so you can use the QST methods.

Alternatively ADF has a module named QUILD that enables calculations through multi-level approaches, and maybe it can be useful.

Another possibility is to run linear transit calculations from reagents and products and guessing the crossing point between the two PESs (of course this is quite rude and approximate).

Hope it was helpful.

Best,

Stefano

Thank you Stefano Racioppi for your answer. I don't have access to the ADF software as it is commercial one. But I am following Enrique Manuel Arpa .

Can you please elaborate the "linear transit calculations ". Are you referring to PES scan from reactant to produce?

Yes,

I meant the scan.

Actually the scan from reactant to product and vice versa (till they converge), in order to have a rough idea of where the crossing point between the two PES is.

Best,

Stefano

Ok thank you Stefano Racioppi . I will also try it

Here is how I got at this problem a while back:

1) Calculate the minimum energy path using Nudged Elastic Band for the ground state potential energy surface of the reactants (in your case, the triplet). The output of this will be the geometries of the reactants (called "beads" in NEB parlance).

2) Take those geometries and run single point calculations on them while enforcing the spin state of the the products (singlet). This will be an excited state for the reactants, but at some point the singlet surface will cross that of the triplet.

3) Taking the geometry of that crossing point bead as the initial state, run another NEB calculation to find the minimum energy pathway to the ground state of the products (singlet).

4) At this point, you have the geometry where the triplet and singlet are degenerate and you have the slope of the potential surfaces in both cases. You can therefore calculate the triplet --> singlet transition probability using the Landau–Zener formula.

Good luck!

Searching the TS and the crossing point (MECP) are two different thinks. If you suspect than the triplet and singlet potential energy surfaces (PES) are close in this region, I recommend finding first the TS both in the S and T PES and the corresponding minima they connect. When you have got this energy landscape of the region start looking for the MECP.

Thank you Agusti Lledos and Adam W. Pelzer for your answers. Now, I am trying to locate TS on the triplet surface.

Agusti gives good advice. Once you've followed that, to locate the MECP you may want to use some automated approach to locate the crossing seam.

Jeremy Harvey developed an automated approach (I think written in c) for this many years ago - his approach is implemented in ADF.

For Gaussian, we have successfully located singlet-triplet MECPs using the python version written by Dan Ess: https://www.chem.byu.edu/faculty/daniel-h-ess/mecp-software-download/

It talks to Gaussian 09 but you can easily modify bits to talk with Gaussian 16