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: [email protected]
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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