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I'm running an IRCs calculation on the TS (transition state) in Gaussian, and I see this table at the end of the calculation with the following header:

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    Summary of reaction path following
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                        Energy    RxCoord
   1                   -0.03023  -3.43818
   2                   -0.02737  -3.09539
   3                   -0.02379  -2.75177
   4                   -0.01970  -2.40792
   5                   -0.01541  -2.06395

I'm wondering how Gaussian calculates that RxCoord parameter. I was searching on the internet but I haven't found anything.

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1 Answer 1

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From reference [1]:

The reaction coordinate is a composite variable spanning all of the degrees of freedom of the potential energy surface.

And from the Gaussian site for IRC documentation:

IRC
This calculation type keyword requests that a reaction path be followed by integrating the intrinsic reaction coordinate [2,3].

From reference [2], the internal coordinates $q$ are defined from the Cartesian coordinates as:

enter image description here

Together with the conditions in equation (6) that assures a zero total momentum and zero total angular momentum motion.

References:
[1] JamesB. Foresman, Aleen Frisch. Exploring Chemistry With Electronic Structure Methods: A Guide to Using Gaussian. 2nd edition.
[2] K. Fukui, “The path of chemical-reactions – The IRC approach,” Acc. Chem. Res., 14 (1981) 363-68. DOI: 10.1021/ar00072a001
[3] H. P. Hratchian and H. B. Schlegel, in Theory and Applications of Computational Chemistry: The First 40 Years, Ed. C. E. Dykstra, G. Frenking, K. S. Kim, and G. Scuseria (Elsevier, Amsterdam, 2005) 195-249.

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