Stress Tensor Information in ChIMES

[Justin D'Souza]

Hello,

I trained a ChIMES model using a force-only fit that works quite well for my purposes. I recently looked to improve the model by re-training it with stresses included in the fit. However, I noticed some inconsistencies between how the different ChIMES codes handle stress information. 

For starters, I noticed that the version of vasp2xyzf.py that I had reads stresses from OUTCAR incorrectly if a frame skip "skipth" is specified. The stresses that are printed for frame number "Nf"  actually correspond to frame "Nf-skipth". 

Also, the stresses are listed in OUTCAR (in kB) in the following order: XX, YY, ZZ, XY, YZ         ZX. But chimes_lsq  prints  (in b-labeled.txt) stresses in a different order: XX, YY, ZZ, XY, XZ, YZ. So the order of the last two components are switched. Finally, the chimes_lsq documentation  (https://chimes-lsq.readthedocs.io/en/latest/lsq_input_file.html#sec-input-files) states that the .xyzf file should contain stresses in the following order: XX, XY, XZ, YY, YZ, ZZ. 

I want to make sure that ChIMES really is reading and fitting the correct DFT stresses. What is the correct way to include stresses? Is this something that has been addressed in a more recent version of the code? 

Thank you!

Best,

Justin D'Souza

[Pham, Huy]

Hi Justin,

 

You have identify the problem: the python script (vasp2xyzf.py) exports the stress tensor components as XX, YY, ZZ, XY, YZ, XZ, while chimes_lsq reads as XX, YY, ZZ, XY, XZ, YZ.

The easiest solution to your problem is to modify vasp2xyz.py to export the stresses as XX, YY, ZZ, XY, XZ, YZ.

 

Let’s know if you have any questions.

 

Best,

 

Huy

 

----------------------------------------------------------

C. Huy Pham

Staff Scientist

Lawrence Livermore National Laboratory

Physical and Life Sciences Directorate

Materials Science Division, L-287

7000 East Ave, Livermore, CA 94550

Phone: (925) 422-7881

 

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