# Confusion about LAMMPS pressure units

Dear Lammps users,

I am confused with the Lammps pressure unit, take the units ‘real’ for example:
force = eV/Angstrom
pressure = bars

However, as we know, pressure (or stress)=force/area. According to the ‘real’ units of force=eV/A, and area unit area=A^2,
the pressure should be in the unit of eV/A^3=160.2 GPa=1.6x10^6 bars. It is much much bigger than bar.

Anyone has a idea where does the puzzle come from? The other units in Lammps have the same problem.

Thanks.

Yunjiang

The units you are referring to are not the LAMMPS’ real units but the metal I think.

According to the documentation :

For style real, these are the units:

• mass = grams/mole
• distance = Angstroms
• time = femtoseconds
• energy = Kcal/mole
• velocity = Angstroms/femtosecond
• force = Kcal/mole-Angstrom
• torque = Kcal/mole
• temperature = Kelvin
• pressure = atmospheres
• dynamic viscosity = Poise
• charge = multiple of electron charge (+1.0 is a proton)
• dipole = charge*Angstroms
• electric field = volts/Angstrom
• density = gram/cm^dim

Otto

Dear Lammps users,

I am confused with the Lammps pressure unit, take the units 'real' for
example:
force = eV/Angstrom
pressure = bars

this is not "real" units.

However, as we know, pressure (or stress)=force/area. According to the
'real' units of force=eV/A, and area unit area=A^2,
the pressure should be in the unit of eV/A^3=160.2 GPa=1.6x10^6 bars. It is
much much bigger than bar.

Anyone has a idea where does the puzzle come from? The other units in Lammps
have the same problem.

this is not a problem. units are always a choice. you can stick with
the "natural" unit for a property or use a more convenient one. the
only difference is a scaling factor. the whole point of the units
command in LAMMPS is to initialize these scaling factors.

in short: it is not a bug, it is a feature.

axel.