Does lammps calculate absolute or gauge presure?

I saw several tutorials and examples that were modeling simple tension tests, or fracture, etc, and when they used npt ensemble they had set pressure to 0. I’m wondering if the pressure in lammps is gauge pressure?

Does lammps calculate pressure in statistical manner like calculating the derivative of Helmholtz free energy vs. volume or etc.?

I want to know if I want to model something at atmospheric pressure, should I simply choose the pressure as 1 (bar) in units metal or 0?

Pressure is computed from virial stress. People use target pressure of 0, because it doesn't make a difference. The pressure fluctuations for condensed systems are several orders of magnitude larger. So choosing 1 or 0 or 1.1 does not make a statistically relevant difference.