[QE-users] Question on Initial and final energies in NEB

[Stella, Martina]

Dear Prof. Baroni,

thanks for prompt reply.

So yes I am using NEB to find a barrier between two states.

Before starting the NEB I have separately relaxed the geometries associated with my two configurations (initial and final). Hence now I have two relaxed geometries.

I start the NEB using the relaxed geometries for these states as first and last image and I use the option to freeze these configurations.

When I start my NEB and I check for the absolute energies of these two images (which supposedly are kept frozen so probably they just go through a SCF cycle?), these energies are quite different from the ones I have from my separate relaxations of the aforementioned geometries . So I was wondering if this is what I should expect.

But maybe I am missing something or thinking about it in the wrong way, I will think about what you said.

Thank again.

Cheers.




Martina Stella

Post-Doctoral researcher

Physics Department, King's College London

Strand,London WC2R 2LS UK


________________________________
From: users <users-bounces at lists.quantum-espresso.org> on behalf of Stefano Baroni <baroni at sissa.it>
Sent: 24 August 2018 12:24:14
To: Quantum Espresso users Forum
Subject: Re: [QE-users] Question on Initial and final energies in NEB

Dear M., I do not quite get what the problem is. The solution may just be a matter of definition. The “Minimum Energy Path” that the NEB is alleged to find is the path in configuration space which *connects two minima* (hence the answer to your question, I guess) and whose maximum energy (along the path) is minimum (with respect to other paths). Put in a different way. What are you using the NEB for? Finding an energy barrier, right? Between what? Stable molecular configurations, right? (hence energy minima).  Does this help? SB

On 24 Aug 2018, at 13:12, Stella, Martina <martina.stella at kcl.ac.uk<mailto:martina.stella at kcl.ac.uk>> wrote:


Hi,

I am new to NEBs calculations and I am getting familiar with the method. I have a couple of questions, maybe trivial, but for which I don't have an answer yet. Hopefully someone can help.

1) How is the energy for the Initial and final structure estimated when I keep the two configurations frozen?
2)Should it be exactly the same as the final relaxed energies that I would get from completely separate calculations with the same simulation conditions and same geometries?
3) If they are not expected to be exactly the same which factors does the difference I may see depend on?

Thanks.


M. Stella
Post-Doctoral researcher

Physics Department, King's College London
Strand,London WC2R 2LS UK


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