[QE-users] Query Regarding Zero Activation Energy in NEB Calculations

[Dyer, Brock]

Hi Aswathy,

I have run a fair few NEB calculations, both in QE and in other programs. Zero energy barrier can mean a few things, but most commonly it boils down to two main reasons.


  1.
The reactant and product geometries are not stationary points on the potential energy surface.
     *
This can happens if the reactant and product are trapped in a local minimum, which nearly every quantum chemistry program struggles to deal with effectively.
     *
Typically you can check if this is the case by examining the vibrational frequencies and see if there are imaginary/negative frequencies. Animating along those frequencies should push you into a more global minimum
  2.
The level of theory you are using could simply not be accurate enough to capture the process you wish to examine.
     *
In my own experience, I know that when I measure single atom transfer in small molecules, the reaction barrier is only about 1-2 kcal/mol, which is extremely small and requires some very very precise calculations. Having used both wB97M-V/def2-TZVPP on ORCA and vdW-DF-C09 with an ultrasoft pseudopotential, I have observed that the former is able to capture the barrier quite well, while the former predicts zero barrier almost every single time.

I am sure that others will weigh in with their own thoughts and experiences, but I figured I would throw in my own just in case it helps. Also, as a side note, are the barriers you get exactly zero, or are they just very very close to zero, i.e. 10^-2 or 10^-3 kcal/mol? I've never encountered a barrier that is exactly zero, so if that is the case it may indicate something different.


Brock Dyer
Ursinus College Department of Physics and Astronomy
Class of 2025


________________________________
From: users on behalf of ASWATHY JAYAPRAKASH
Sent: Tuesday, March 11, 2025 4:26 AM
To: users at lists.quantum-espresso.org
Subject: [QE-users] Query Regarding Zero Activation Energy in NEB Calculations

Dear Quantum ESPRESSO Community,


I am encountering an issue with NEB (Nudged Elastic Band) calculations. In some cases, I am getting zero activation energy either for the forward reaction or the backward reaction. Could anyone please clarify what this means? Does this indicate a problem with my input files or calculation setup?

I am following the same input structure that worked well in other cases, where I was able to obtain reasonable activation energies. However, in these particular cases, I consistently get zero barriers.

Any insights or suggestions would be greatly appreciated.

Thank you in advance for your help!

Best regards,

Aswathy Jayaprakash

Research Scholar

CUSAT
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