Polymer deformation and fix_npt

Dear LAMMPS users,

I have a question about deformation simulations; having read on this topic, I have noticed that the best way to deform a chain is to use fix_deform with fix_npt; however, after equilibrating the chain and shrink-wrapping the simulation box, I use periodic boundaries and deform the chain in x direction.

I do the following:

fix 1 all npt temp 1 1 1 y 0 0 1 z 0 0 1 drag 1
fix 2 all deform 1 x scale 2 units box remap x

However,my temperature is becoming high (from 1 to 26 in reduced units) which is not what I was expecting. I am using reduced units, and run the simulation for 300000 steps, with a timestep of 0.01.

Am I missing something obvious?

With best wishes,

In LJ units, a step of 0.01 is big. When you
deform the box you are typically putting energy
into the system. If you do it too rapidly
the temperature will rise, despite the thermostat.


Dear Steve,

Thanks, that’s very helpful- I will go for smaller steps!