Pxx Pyy too high

Dear Lammps Users,

I am simulating a crystal periodic in two directions that is x and y and non periodic in z. I am using NVT for equilibrating the crystal at 300 K. I observe that Pzz fluctuates around zero which is expected as there is vacuum above and below the crystal in z direction. But Pxx and Pyy are as high as 20000 bars when NVT ensemble is used.

I thought the crystal will expand (to achieve mechanical equilibration) and pressure will be zero during equilibration. But I get a very high value of pressure Is there a physical reason why crystal is not expanding or is the pressure calculation not correct?

I would be very thankful if someone can help me to clarify my doubt.


Nikhil Joshi

Deark Nikhil,

The nature of the NVT ensemble is that N,V, and T define the macrostate of your system and are not varying. Since you are in the NVT ensemble, the crystal cannot change size in the Pxx and Pyy directions to relieve the stress. It sounds like your goal is a macrostate defined by Pxx=Pyy=0, rather than Lx,Ly=constant. To do this you want to simulate in the NPT ensemble.


Thomas O’Connor