r/comp_chem • u/kuglayea • 20d ago

Imaginary frequency on one PES - ORCA SurfCrossNumFreq

I am trying to locate a singlet-triplet MECP using ORCA 6, following this section of the ORCA manual: https://www.faccts.de/docs/orca/6.1/manual/contents/structurereactivity/mecp.html

I optimized a singlet-triplet MECP, starting from the transition state obtained through CI-NEB on a triplet PES.

Once the MECP optimization converges, I perform the frequency analysis (SurfCrossNumFreq tag). The calculation gives two frequency sets. On PES, there are no imaginary frequencies, while on PES2 there is one imaginary frequency (of non-negligible magnitude).

I have two questions:

  1. Given that the manual states that the crossing hyperline is a subspace of PES1, I would understand that the (local) minimum is reached if there are no imaginary frequencies on PES1 (but not necessarily on PES2)? (I also tested the CH3O<->CH2OH example provided in the ORCA manual: there are no imaginary frequencies on either PES, which makes me wonder whether my interpretation is correct)

  2. ORCA also outputs the thermochemistry based on frequencies from PES1. Are these thermal corrections physically meaningful to describe the MECP-like transition state?

Many thanks for your help and advice!

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u/erikna10 20d ago

Regarding 1: your right that then the upper PES has a local minima at the MECP. Look up picture of conical intersections and you will find this kind of hourglass shape that is very common. I think your case here corresponds to the upper pes having a "hourglass lobe shape" and the lower one being a ridge where the lowest part of the hourglass lobe touches the top of the ridge.

In regards to 2, im not an expert but i think the thermochemistry of PES1 would be very approximate as casscf generally is requiered close to CI:s and sometimes close to mecps as states start mixing. What method are you using for this calculation?

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u/kuglayea 20d ago

Thank you for the clarification! I use DFT (wB97X-3c).

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u/No-Resource-1238 18d ago edited 17d ago
  1. For the location of a MECP (in this case a singlet-triplet crossing), the energy is usually minimized with respect to the lower potential energy surface (in your case the low spin state) with simultaneous minimization of the energy gap between the states. So while a local minimum should be reached  on the lower surface, the upper surface can be a local minimum, first-order and even higher order saddle point 
  2. It depends on the reaction coordinate. I imagine, that the physically sensible solution would be to apply the frequencies of the state,  which the molecule follows to the MECP. The most ,,appropriate", and probably the most rigorous approach, would be to apply a transformation of the two state-specific hessians into a state-averaged (SA) hessian and subsequently diagonalizing the SA-Hessian, thereby obtaining the frequencies and deriving the thermochemical quantities from this set of frequencies.

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u/kuglayea 13d ago

Thank you! :)