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

[Stefano Baroni]

Sounds like something wrong is going on. You may want to post your input files and some relevant snapshots of the output, although I am afraid that at that point others may be more knowledgeable than me … SB

> On 24 Aug 2018, at 13:57, Stella, Martina <martina.stella at kcl.ac.uk> wrote:
> 
> 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 <mailto:users-bounces at lists.quantum-espresso.org>> on behalf of Stefano Baroni <baroni at sissa.it <mailto: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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