Re: [QE-users] oscillations in G(V) diagrams

[Lorenzo Paulatto]

Hello Jakob,

you do not give much detail about what you are doing. As said, QHA is 
effective when dealing with one degree of freedom (i.e. cubic 
semiconductor) but harder to use in any other case. Are you doing a 
constant-volume variable-cell relax (aka cell_dofree='shape') for a 
series of volumes? And then computing for each volume the Helmholtz free 
energy (from the phonons directly or from the dos)? Are you doing this 
your self by hand or using one of the available QHA software packages?


The oscillations could be an honest bug, or a difficult integration of 
the DOS.


cheers


On 04/11/2022 09:20, Jakob Kraus wrote:

Dear QE users,


I am trying to calculate heat capacities, and to this end, I have 
calculated the Gibbs energy as a function of volume


at several temperatures in a range of 0-700 K.

However, the functions that result from this do not show a single 
minimum, but rather a whole lot of oscillations, which


seem to originate from the phonons, since the E(V) diagram at 0 K 
shows a single minimum as expected.


I am thankful for any help you can offer with getting rid of these 
oscillations.


I have attached input files and some plots to showcase the problem.


Best regards,


Jakob Kraus, TU Freiberg

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Dr. Lorenzo Paulatto
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Re: [QE-users] oscillations in G(V) diagrams

[Lorenzo Bastonero]

Dear Jakob,

Do you mean the Helmoltz free energy (F)? 

As by definition, the Gibbs free energy (G) does not depend upon volume, but on 
pressure and temperature.
At p=0, then they are the same, but notice always that:

G(p,T) = min_{V}[F(V, T)+pV] 

Thus, at p=0, you still need to take the minimum of F at the fixed temperature.

Moreover, I guess you are using the quasi-harmonic approximation. There exist 
several flavours. Unless your system is isotropic, scaling the volume 
isotropically can be a source of error (due to a physical assumption, not due 
to the numerics).

Best,
Lorenzo Bastonero


Lorenzo Bastonero

PhD Student
U Bremen Excellence Chair,
Bremen Center for Computational Materials Science,
and MAPEX Center for Materials and Processes

University of Bremen
Faculty of Production Engineering
TAB-Building, Room 3.32
Am Fallturm 1
28359 Bremen, DE

http://www.hmi.uni-bremen.de/

> Il giorno 4 nov 2022, alle ore 09:20, Jakob Kraus 
>  ha scritto:
> 
> Dear QE users,
> 
> 
> I am trying to calculate heat capacities, and to this end, I have calculated 
> the Gibbs energy as a function of volume
> 
> at several temperatures in a range of 0-700 K.
> 
> However, the functions that result from this do not show a single minimum, 
> but rather a whole lot of oscillations, which
> 
> seem to originate from the phonons, since the E(V) diagram at 0 K shows a 
> single minimum as expected.
> 
> I am thankful for any help you can offer with getting rid of these 
> oscillations.
> 
> I have attached input files and some plots to showcase the problem.
> 
> 
> Best regards,
> 
> 
> Jakob Kraus, TU Freiberg
> ___
> The Quantum ESPRESSO community stands by the Ukrainian
> people and expresses its concerns about the devastating
> effects that the Russian military offensive has on their
> country and on the free and peaceful scientific, cultural,
> and economic cooperation amongst peoples
> ___
> Quantum ESPRESSO is supported by MaX (www.max-centre.eu)
> users mailing list users@lists.quantum-espresso.org
> https://lists.quantum-espresso.org/mailman/listinfo/users

___
The Quantum ESPRESSO community stands by the Ukrainian
people and expresses its concerns about the devastating
effects that the Russian military offensive has on their
country and on the free and peaceful scientific, cultural,
and economic cooperation amongst peoples
___
Quantum ESPRESSO is supported by MaX (www.max-centre.eu)
users mailing list users@lists.quantum-espresso.org
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