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This question is regarding Gaussian Computational software package.

I have a system where one product forms from one conformational arrangement of molecular intermediate (conformational minimum) and another product forms from a second conformational arrangement of the same molecular intermediate (another conformational minimum). These two arrangements can be interconverted via a rotation about a single bond. In order to investigate the subsequent TS from each pathway, I also need to find the pathway that interconverts the two conformers.

How can I use Gaussian to find the rotational transition state that converts these two conformations? I have tried QST2 method using both conformers as input but did not work; I also tried doing a scan along the rotation about the single bond and do a subsequent TS search from the scan maxima but to no avail. Any other methods that I can use to find rotational transition states? I imagine the PES to be very flat, how can I improve my input for tighter convergence?

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  • $\begingroup$ Could you be more specific on what your problems are? Namely, when you say that QST2 "input but did not work", what do you mean? What exactly happened? The same applies for PES scan. $\endgroup$ – Wildcat Oct 28 '16 at 11:34
  • $\begingroup$ Remember that for a QST2/3 scan to work, the atoms have to be ordered the same in each input block. Did it not work, because it aborted with an error, or did it not work because it did not lead to a stationary point? Please post an example of your input including keywords. Information on the version might also be necessary. In general such questions are not really on topic here though. I don't remember if gaussian has a user forum, but there would still be ccl. $\endgroup$ – Martin - マーチン Oct 28 '16 at 12:13
  • $\begingroup$ Hi Wildcat and Martin, the qst2 outputs with normal termination; it is however not a product that i (or any chemistry) expect(s) as it forms strange ring fusions. It could have been that I did not order the atoms correctly in the same sequence in the two input structures. I will check this. Thanks! $\endgroup$ – X Zhang Oct 28 '16 at 15:53

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