nve increasing temperature after nvt

Dear LAMMPS Users,

I am trying to run a very basic simulation and have encountered a problem with nvt and nve. When I apply a fix nve after an nvt the temperature of the system increases.

More specifically I am simulating a thin solid film of 32800 atoms. I have fixed the bottom most atomic layer on the z-axis (800 atoms):

fix f fixed setforce 0.0 0.0 0.0

The I use the command

fix NVT mobile nvt temp 300.00 300.00 0.04 tchain 4 tloop 50

where mobile corresponds to the atoms that have not been fixed. I then unfix nvt and apply a fix nve for 2 ns.

The problem I encounter is that the temperature rises about 10-20 K when I apply a fix nve.

Does anyone have any input or suggestions?

Thank you,


What group was fix nve applied to, all or mobile? How did you estimate the temperature?


Thank you so much for getting back to me. I applied fix nve to the mobile atoms and I used the command “compute Tmobile mobile temp” to compute the temperature of the mobile atoms. I’ve included my script below.

Note I have other atomic groups as I later want to create a hot and cold junction.

Thanks again,



10-20 K increase in temperature seemed fine to me. Your timestep size is already small, so I would recommend you to also minimize the volume of the supercell by adding fix box/relax. You might be able to reduce the temperature increase a bit.