Allow non-conservative simulations without stress

[PicoCentauri]

This is a bit delicate. The problem seems to be that the vflag_global is kind of always set. I tried a minimal NVE simulation and a thermo_style where there is no pressure output. But even during this run, the flag is set to true.

The only workaround I see is to warn the user at init time and unfortunately silently ignore the pressure output during the run.

I am open to any better suggestions. @Luthaf or @HaoZeke.

[Luthaf]

If vflag_global is set, I think we should compute the virial. One solution if the model does not have non_conservative_stress would then be to fallback to backward propagation for this. So if the user sets non_conservative on, we try to use both nc_forces and nc_stress, falling back to conservative forces/stress if one of them is missing. If both are missing it should be an error IMO.

Alternatively, we could allow non_conservative forces/non_conservative stress in the lammps input to only use one of them.

I don't have strong feeling here, we can either automatically use autodiff when the non_conservative output does not exists; or force the user to tell us and have non_conservative on mean we use non-conservative for everything.

[PicoCentauri]

I think a fallback to autodiff and write a warning might not be read by the user. I am more in favour of raising an error if either non_conservative_forces or non_conservative_stress is not existing and forcing them to manually set the value of non_conservative. We could go for the options as you suggest @Luthaf:

1. non_conservative_forces - stress and forces are nc
2. non_conservative_force - forces are nc, stress is ad
3. non_conservative_stress - forces are ad, stress is nc

[Luthaf]

I'm fine with the error, but it seems that there is a typo (_ instead of ) in your example? I can see the following options in the LAMMPS input file:

• non_conservative off default, do not use any non-conservative output
• non_conservative on use both nc stress and forces
• non_conservative forces use nc forces and conservative stress
• non_conservative stress use nc stress and conservative forces

[PicoCentauri]

Yes sorry for the typo. Okay let's do this options.