Dear developers and users of Lammps, my pace potential works poorly on stress prediction.

**Potential Creation**

I have created a database of 1000 DFT configurations for Cu to train an ACE potential. I’ve got 8 atoms per configurations so this test simulation is fast. After the fitting, I get a RMSE of 1.1 meV/at on energy and 3.6 meV/ang on forces which seems to indicate this ACE potential is able to represent the thermodynamical state pretty well.

**The problem**

Then, I’ve done a lammps calculation on the relaxed Cu system with 0 velocity on every atom and 0 step using this PACE potential. I’ve noticed the pressure is -80.45 GPa while the DFT pressure is 1.58 GPa. I find it strange that the pressure is that high since the highest DFT force on an atom is 1.6e-5 eV/ang and the pace potential predicts even lower forces. For the 3000 pressures, it varies between 80 and 88 GPa as if there was a constant to remove.

**What I would expect**

Although the effective force of an atom isn’t directly related to the stress, I would expect the stress to be closer to the DFT result as the pace potential seems to describe the interaction correctly.

**Questions**

- Is the pressure expected to works well using pace as there is only fitting on energy and forces ?
- Is there a way to output the different quantities used to compute the virial pressure for non-linear potential ?
- How could I get more insight on pace pressure ? Note that I could not find much information about pace pressure in the litterature.
- Is there a documentation about how Lammps compute virial pressure for non-linear potential ?

**Additional Information**

- As the pace potential is anharmonic, I cannot compute pair interaction to get dx, dy, dz, fx, fy, fz and manually compute the virial stress.
- I’ve substracted the energy of a free Cu atoms from the energy in my database
- The high pressure comes from the pressure pair term and the contribution seems equal from every atom.
- I’ve been careful with the units, as I’m using metal units which means pressure in bar.
- I don’t think the 8 atoms per configuration are the problem as I’m looking at a configuration
- I’m using the version of Lammps from the 2 Aug. 2023.

**A bit of code**

Here are the two line I use to get the virial pressure. This pressure is the same as the pressure in the log file (thermo) since the velocity are set to 0.

```
compute svirial all pressure NULL virial
fix svirial all ave/time 1 1 1 c_svirial file svirial.out mode vector
```

Thank you in advance