Thermal Expansion using MLFF

[Fermi1976]

Dear VASP Forum,
I am interested in calculating the thermal expansion coefficient of graphite using MLFF, so I can utilize the temperature-dependent lattice constants later on to perform temperature-dependent phonon calculations. I have been trying using NPT, but I have some issues, I will be grateful if you could give some advices how to improve the calculations.

I am attaching the INCAR ML_LOGFILE files. I used 288 atoms, 3x3x3 k-mesh.
I am attaching also, the Bayesian error, RMSE, volume, Temp and c-lattice constant as a function of MD steps.

My questions are as follow:
1- There is a sharp drop in the volume (mainly due to the drop in c-lattice constant). This happened in the training stage (below 6000 MS steps). I appreciate your feedback for this issue
2- The number of the AB configurations is 94, how can increase this number to improve the quality of the FF?

Thank you

[ferenc_karsai]

Can you please send us all your files even if they are big (XDATCAR, POSCAR, POTCAR, ML_AB/N, ICONST, OUTCAR).
I would like to have a look at the trajectory, have you done so?

I already saw a type in your INCAR file. You wrote "Ecut = 600". I guess you meant "ENCUT = 600".

What you can try in the meantime:
-) Use ENCUT= 600 instead of the typo.
-) Try a different PMASS.
-) Try a different van der Waals (IVDW=10).

But these are only guesses, so I'm not sure any of these will help you.

[Fermi1976]

1- Thank you for finding the Ecut typo. I am repeating the calculations using ENCUT=600. I doubt this will make significant difference since ENMAX =400 in OTCAR and I am using PREC= Accurate (==> ENCUT ~520 eV).
I am wondering why VAPS did not show any warning of this typo.

2- I tried different PMASS values (1000, 2000, 3000), I did not get noticeable difference.

3- I tested different IVDW, IVDW=10 gave c = 6.39 A, which is far from the experimental value of 6.67 A. Whereas using IVDW=12 shows better prediction of c- value (6.66 A).

4- I will find away to send you the files you requested, they are so big.

Thank you again, I appreciate your willingness to help.

[Fermi1976]

Hi ferenc_karsai,
Could you please send me your email so I can share with you a link to access the files?

Regards,
Iyad