[QE-users] Energy for Cells of Different Charge

[Tamas Karpati]

Dear Robert,

I guess your point is in characterizing the interaction between ionic
species A and B.
If you put them together in a single cell (ie. A + B) you have a good quantum
chemical/physical description of the interaction. In the better case
you can even
simulate the charge neutral systems in a cell. Since you have an atomic level
model, the interaction is a "sum" (+ more) of **many** atom-atom interactions.
And this makes it difficult for you to extract a simple expression for
the general case.

Although such interactions can be abstracted to a higher level by eg. some
incremental models, they will be of lower accuracy (and thus predictive power).
I'm afraid you cannot save doing the simulations for each specific case.
Of course, you may be lucky to fit association energies on these results
successfully, but you'll need to be careful to account for the applicability of
your simplified model as it will be correct for just a limited no. of cases.

Regards,
  t




On Tue, Dec 29, 2020 at 4:04 PM Robert Stanton <stantor at clarkson.edu> wrote:
>
> Dear Tamas Karpati,
>
>     Yes definitely sorry for the unclear original question. The E(a+b) - [E(a) +E(b)] quantity is the one of interest here. My concern though is that the reference for total dft energies here matters as it crops up in the finals result for the formula above, even if all references are the same. It seems from trying several things that this is ends up being problematic in the case of cells with different charge specified.
>
>    I know the QE faqs say reference levels are not well defined when using PBCs, but is there any way to try to extrapolate to vacuum (all cells are same size with a lot of vacuum in all directions) or am I maybe missing something in the approach and something else is the problem? Thanks again for the information!
>
> Regards,
> Robert Stanton
> Graduate Student
> Clarkson University
>
> On Tue, Dec 29, 2020 at 3:16 AM Tamas Karpati <tkarpati at gmail.com> wrote:
>>
>> Dear Robert,
>>
>> Thanks for clarification. If I understand correctly,
>> you intend to model A + B interaction energies using
>> E(A) and E(B) values obtained for the separate systems.
>>
>> Separate means non-interacting so you lost your
>> interaction energy in such simulations.
>> At the atomic scale the situation is more complex
>> than it is for eg. point charges (think of the distributed
>> electron and shielded core charges at small scales).
>> From this point of view, your current problem is also
>> different to the separation of vdW parameters into
>> atomic (~ isolated) parameters where they model
>> the interaction of point-like objects. Yours is not one.
>>
>> Most probably you need the association energy
>>      E(A+B) - [ E(A) + E(B) ]
>> evaluated for carefully selected cells and params.
>>
>> (Sorry for the inconvenience: last time E meant a cell
>> or material, here it denotes total energy.)
>>
>> Bests,
>>   t
>>
>> On Tue, Dec 29, 2020 at 1:33 AM Robert Stanton <stantor at clarkson.edu> wrote:
>> >
>> > Dear Tamas Karpati,
>> >
>> >      Definitely, the question is basically what association energies are for things which are bound ionically. It seems I require calculation of the lone ions with compensating background charge, as well as the neutral compound. I am just wondering how I can partially correct/change what these absolute DFT energies are with respect to for valid comparison between calculations. Is there a way to do this or do you think even if achieved, this still might be a problematic approach? Thanks in advance!
>> >
>> > Regards,
>> > Robert Stanton
>> >
>> > On Mon, Dec 28, 2020 at 12:20 PM Tamas Karpati <tkarpati at gmail.com> wrote:
>> >>
>> >> Dear Robert,
>> >>
>> >> Since each member of the series E(2-) ~ E(1-) ~ E ~ E(1+) ~ E(2+)
>> >> etc... dwell on a different
>> >> potential energy hypersurface, there is no direct physical meaning of
>> >> their energy differences.
>> >> Can you give the concrete question you try to answer?
>> >>
>> >> On the other hand, it can be quite useful to compare eg. the
>> >> stabilization energies (or other
>> >> derived energy-like quantities, even band positions). In case of the
>> >> stab. ens. it is common to
>> >> make hypothetical reactions and evaluate their energies which can then
>> >> directly be compared.
>> >>
>> >> Hope this helps,
>> >>   t
>> >>
>> >>
>> >> On Mon, Dec 28, 2020 at 7:23 PM Robert Stanton <stantor at clarkson.edu> wrote:
>> >> >
>> >> > Dear all,
>> >> >
>> >> >     I am just wondering if there is, in general, an approach to comparing energies between two cells with different tot charge specifications? Is there a way for example to extrapolate the energy values to be w.r.t. vacuum to have a common comparison point?
>> >> >
>> >> >     I have seen somewhat similar questions, generally these are specifically for 2D system, where it's stated there are examples addressing this, but have not been able to find those examples in the QE install. Any information, or a pointer to this specific example (if it applies to more than just 2D) would be greatly appreciated! Thanks in advance for any help.
>> >> >
>> >> > Regards,
>> >> > Robert Stanton
>> >> > Graduate Student
>> >> > Clarkson University
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