Sign issue in stress calculations

[amirhajibabaei]

Theoretically
Stress=stress1 + stress2,
where
stress1 = (sum over all atoms F.T@r) / volume
where r is coordinates and F forces on atom.
stress2 is related to derivatives wrt cell.
This works fine in ParametricCalculator (test using calc.calculate_numerical_stress(atoms)).
But in AutoForceCalculator (machine learned) we have to multiply stress1 by -1 in order to get the right numerical stresses.
Everything seemingly works just fine but I still can't explain the -1 multiplyer.