# negative pressure

Dear Lammps users,

I have a quick question about how lammps calculate the pressure.
I am using “real” unit in my simulation.
I am encountering some negative pressure results during my simulation.
I just want to know is the pressure is base on “absolute” or “gauge” pressure ?
Just curious if negative pressure is acceptable or it is just a diverging solution.

Thanks,
Erik

Dear Lammps users,

dear erik,

I have a quick question about how lammps calculate the pressure.
I am using "real" unit in my simulation.
I am encountering some negative pressure results during my
simulation.
I just want to know is the pressure is base on "absolute" or "gauge"
pressure ?

negative pressure is quite possible.
the sign of the pressure indicates
the direction of the pressure on the
"walls" of a periodic cell. a system
with negative pressure wants to shrink;
a system with positive pressure wants
to expand.

Just curious if negative pressure is acceptable or it is just a
diverging solution.

no, this is quite normal. however,
you have to decide whether it is
compressibility in dense systems is not
very high, so pressure can build very
quickly.

cheers,
axel.

Dear Dr. Kohlmeyer,

I guess then it is acceptable if the pressure is negative after minimization process.
However, I am confuse because during my equilibration process at constant T = 2500K, my pressure started positive (3000 atm) and went to negative (-30 atm)at some point and goes back to positive.
I am using NVT ensemble at T = 2500K for methane oxidation (1CH4 + 100O2) simulation.

I read a restart file to start the simulation. The restart input already went through minimization, heat rampup (0.5K to 2500K), equilibrated for 10ps (100000 iterations with 0.1fs timestep - no negative pressure detected during this equilibration process).

I attach my input file and thermo output. At Step 290000, the pressure suddenly goes to -29.424578. I am curious is this diverging solution or not.

Thanks,
Erik Winardi

in.genconfig (1.86 KB)

out.txt (4.95 KB)

Dear Dr. Kohlmeyer,
I guess then it is acceptable if the pressure is negative after minimization
process.
However, I am confuse because during my equilibration process at constant T
= 2500K, my pressure started positive (3000 atm) and went to negative (-30
atm)at some point and goes back to positive.
I am using NVT ensemble at T = 2500K for methane oxidation (1CH4 + 100O2)
simulation.
I read a restart file to start the simulation. The restart input already
went through minimization, heat rampup (0.5K to 2500K), equilibrated for
10ps (100000 iterations with 0.1fs timestep - no negative pressure detected
during this equilibration process).
I attach my input file and thermo output. At Step 290000, the pressure
suddenly goes to -29.424578. I am curious is this diverging solution or
not.

have you checked, whether this correlates to some
structural changes and changes in kinetic/potential
energy, e.g. a reaction happening?

axel.