## Rate constant calculation - electronic energy barrier or Gibbs energy barrier?

Vasp transition state theory tools

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pecher
Posts: 13
Joined: Wed Oct 29, 2014 1:51 pm

### Rate constant calculation - electronic energy barrier or Gibbs energy barrier?

Dear Graeme and others at the UT,

I was wondering: If I calculate the reaction rate with your dymrate.pl script (or by using the equations from your JCP 124, 044706 (2016) paper), do I use the electronic energy barrier or the Gibbs energy barrier? I have a system where Gibbs energy corrections are essential (I use the harmonic oscillator partition functions), but I'm not entirely sure if your rate equation already contains these corrections and if I would therefore double count them if I insert the Gibbs energy barrier into the equation.

Thank you and best wishes,
Lisa
graeme
Posts: 1999
Joined: Tue Apr 26, 2005 4:25 am
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### Re: Rate constant calculation - electronic energy barrier or Gibbs energy barrier?

The dymrate.pl script includes the harmonic entropy of the minimum and saddle. The barrier should be the enthalpy difference between the saddle and minimum, without entropic contributions. If you need to go beyond the harmonic approximation, you will also need another way (e.g. sampling) to calculate the free energy difference between the minimum and transition state.
pecher
Posts: 13
Joined: Wed Oct 29, 2014 1:51 pm

### Re: Rate constant calculation - electronic energy barrier or Gibbs energy barrier?

I just wanted to let everyone know that I looked at all the equations in more detail and found out that you need to take only the electronic energy difference if you use the script or the equations from the JCP paper (which is from 2006, there was a typo in my previous post). These equations are equivalent to the formulation k(T) = (kB T)/h * exp(-Ga/(kB T)). So if you take the Gibbs energy in Graeme's equation, you will have the difference between Ga and Ea included twice!