# [lammps-users] Negative pressure in NVT simulation for a liquid alloy

Dear friends,

I am studying a binary liquid alloy Cu-Zr using EAM potential. In order to get the liquid configuration, I start with intial fcc configuration and then obtain the liquid configuration at the desired temperature (say, 1900 K). I have set the pressure to be zero. When I monitor T, P, V during the lammps run, I find everthing to be alright. T is const, Avg pressure is 0, volume shows flucturations about some mean value. There is no systematic drift in T, P, V as well as the total energy after a long simulation run.

I record the configurations during the NPT simulation for further use in NVT simulation. I intend to run NVT for studying the structure and dynamics of the alloy. When I start with the one of the configurations recorded during NPT, I find that the average value of the pressure is negative. My question is whether it is alright. Is getting negative pressure in my case wrong? I am confused. Could you please explain.

With regards,

|

Dear friends,

I am studying a binary liquid alloy Cu-Zr using EAM potential. In order to get the liquid configuration, I start with intial fcc configuration and then obtain the liquid configuration at the desired temperature (say, 1900 K). I have set the pressure to be zero. When I monitor T, P, V during the lammps run, I find everthing to be alright. T is const, Avg pressure is 0, volume shows flucturations about some mean value. There is no systematic drift in T, P, V as well as the total energy after a long simulation run.

I record the configurations during the NPT simulation for further use in NVT simulation. I intend to run NVT for studying the structure and dynamics of the alloy. When I start with the one of the configurations recorded during NPT, I find that the average value of the pressure is negative. My question is whether it is alright. Is getting negative pressure in my case wrong? I am confused. Could you please explain.

your mail is lacking some important details:
- _how_ negative is the pressure?
- what box dimensions did you use for NVT?
instantaneous or average?

axel.

When I start with the one of the configurations recorded during NPT, I find that the average value of the >pressure is negative.

During the NPT simulations, presumably the pressure was osciallating around 0.0.
If the snapshot you chose happened to have a negative pressure, then
the time 0 (initial) snapshot of the NVT run should print out the same pressure,
modulo small round-offs if you are using imprecise atom coords to restart.
Note that pressure has a kinetic component, so if your restart did not
also preserve velocities, then you will not get the same initial pressure in the
NVT as in the final NPT snapshot.

Steve