Good morning,
we are conducting simulations of liquid metal surfaces using the VASP MLFF. Currently, we are benchmarking the results of trained MLFFs for several systems (e.g., liquid Ga, Sn, GaPt, etc.) by comparing them to direct AIMD simulations with VASP.
One observable, which is also of high interest for our experimental colleagues, is the surface tension of those systems. Therefore I want to compare surface tensions of DFT and MLFF simulations for the same system at a range of different temperatures.
I tried to calculate them with the usual formula (with vacuum introduced in zdirection): r = 0.5 * Lz * (Pzz0.5*(Pxx+Pyy)), where Pij are the respective components of the pressure tensor (where I assumed that I can use the stress tensor from VASP for this purpose).
I noticed, however, that the printouts concerning the stress tensor are quite different in the OUTCARs of AIMD and MLFF simulations.
In the AIMD calculations (for a pure Ga surface slab), the OUTCAR section in each MD step is:
FORCE on cell =STRESS in cart. coord. units (eV):
...
Total 20.47972 35.62209 24.26207 4.37477 3.16535 0.29312
in kB 5.63510 9.80159 6.67583 1.20374 0.87096 0.08065
external pressure = 7.37 kB Pullay stress = 0.00 kB
In the MLFF calculations of the same system, the respective OUTCAR section is (I assume at least):
stress matrix after NEB project (eV)
16.76946 0.78240 4.79830
0.78240 28.29533 3.93928
4.79830 3.93928 19.15916
My question now is, how I can compare the values of both calculations directly, i.e., obtain pressure tensors from both models for further evaluation and calculation of the surface tension.
In the AIMD case, there is a `FORCE on cell =STRESS` note at the beginning, so I assume that I need to take the values of the line `Total 20.47972 35.62209 24.26207 4.37477 3.16535 0.29312` given in eV and then invert their sign?
In the MLFF case, has the comment ` stress matrix after NEB project` any influence on the comparability, since no such `NEB project` is noted in the AIMD case?
Thank you in advance!
Best wishes,
Julien
Stress Tensor comparison DFT/MLFF
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Re: Stress Tensor comparison DFT/MLFF
Strange, I thought you would find the stress tensor written in the OUTCAR file both when running ML or AIMD.
Could you share your INCAR, POSCAR, POTCAR, KPOINTS, and OUTCAR files for the ML and AIMD runs?
(if you can show the issue on a system with few atoms then better)
Could you share your INCAR, POSCAR, POTCAR, KPOINTS, and OUTCAR files for the ML and AIMD runs?
(if you can show the issue on a system with few atoms then better)

 Newbie
 Posts: 22
 Joined: Wed Feb 23, 2022 10:18 am
Re: Stress Tensor comparison DFT/MLFF
Ah sorry, you are right! I overlooked the respective "stress matrix after NEB project (eV)" section in the AIMD OUTCAR files.