Confusion about LAMMPS pressure units

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1

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

2

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

3

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.