# [Pw_forum] variable cell MD target pressure

luisen luisen at metodos.fam.cie.uva.es
Thu Oct 24 10:14:53 CEST 2013

Dear Davide,

Thank you very much for your rapid answer. I have just one quick thought
about the consistency between the pressure obtained from several NVT runs
and the result obtained directly from the stress tensor.

Did you use the "soft" cutoff method? (ecfixed, qcutz, q2sigma stuff)
Did you include in the calculation of the derivative the kinetic energy?
Do you think the entropic term of the Helmholtz free energy may have
an important impact?

Sorry if the questions look stupid, it is just the first thought that came
to my mind.

I have another question. In liquids it doesn't really make sense to change
the shape of the simulation cell. It is only the volume that matters.
Moreover, for certain magnitudes it is more convenient to have cubic
cells because of the increased degeneracy in modules of k-vectors in
reciprocal space, which helps with statistics. However I could not find
a switch in the input parameters that changes the 3 axes in the same
amount, so as to preserve the shape of the box. It is possible to keep it
orthorhombic, but not cubic. Is there any way to do that without modifying
the code ?

Yours,

Luis Enrique

> Dear Luis,
>     I'm the culprit who coded output of the kinetic stress, but
> without adding it to the total stress. Are you sure it is added
> to the total stress?
>
> The issue is that it is not clear whether the "Ion kinetic term"
> should be there. In my opinion it should be added to the total stress,
> as it is done in classical MD and in the CPV code.
>
> However, I tried to perform NVT MD runs of a liquid metal at different
> volumes, and to compare by d^2<E>/dV^2 with the average stress computed
> with and without the ions kinetic term. Unfortunately results were
> inconclusive.
>
> Therefore, in order not to break the behavior of the code, I decided to
> print it, without summing it to the total stress.
>
> Comments on this topic are very welcome.
>
> Best,
>     Davide
>
>
>
> On 10/23/2013 10:21 AM, luisen wrote:
>>
>>
>>
>>   Dear all,
>>
>>   I am writing to ask for confirmation about a "strange" feature in
>>   the program, that concerns molecular dynamics calculations at finite
>>   temperature (non-zero) with variable cell.
>>   As far as I have experienced it seems to me that the target pressure
>>   that one states in the input file is the total pressure, I mean, including
>>   the kinetic term coming from the motion of the ions due to the temperature,
>>   while the stress tensor and pressure writen in the output file includes only
>>   the potential terms, but not the kinetic ones.
>>
>>   This message is just to request a confirmation from the developers that I am
>>
>>   Note that this feature is very confusing for users; I could spot it because
>>   the system I am studying is liquid Be, which has a very high temperature and
>>   a very high number density, so that the kinetic part of the pressure is 20
>>   kbar. So it is really confusing asking for 0 kbar and obtaining
>>   oscillations around -20 kbar in the output file.
>>   May I suggest to modify it in the future release of QE?
>>
>>   Yours,
>>       Luis Enrique Gonzalez