I am a PhD student and I am working on some atomistic simulations using LAMMPS. I am working on crystallin materials.
I performed some 2D simulations of a crystallin system, which is heated at a certain temperature. The system is kept at constant temperature and the microstructure evolves.The evolution of the system is such that it reduce its volume during the simulation.
I performed in NPT ensemble using the ‘fix npt’ command. I used a lennard jones potential for interactions, with coefficients 8 - 4 ( ‘pair_style nm’ in lammps). I started my simulations from a previously relaxed configuration and assigned velocities at time t=0 with the ‘velocity’ command.
Simulations run well, I kept an eye on the values of temperature and pressure which fluctuates but in a limited range of values.
For some reasons, I was interested in trying to see the system evolution in canonical conditions, NVT ensemble, with a langevin thermostat. Actually, I tried 2 different thermostats:
Nosé - Hoover
In the NPT case, the system changes its volume, so I was expected to see pressure changing in the NVT case;
I performed some simulations with the NH thermostat, trying different values of the damping term. In all the cases, I get similar profiles for the temperature and the pressure during the simulation. Pressure is oscillating, it is not zero but in any case it is not really high.
In conclusion, choosing a reasonable value for Tdamp I was able to kept the temperature constant and I get similar profiles for the evolution of pressure during the simulations.
Difficulties arised with the langevin thermostat.
First question, I don’t know if it is reasonable to assign velocities to atoms at the begging or just to set all the velocities equal to zero and then let the thermostat acting. What I noticed is that, if i follow this last procedure, I have to set a value of the temperature in the thermostat lower than the one desired. Let’s say: for having a final value of temperature of 0.2 (reduced units) I have to set Tstart=Tstop= 0.13 in the ‘fix langevin’ command.
Moreover, pressure is behaving in a really strange way. It changes its value in a continous way, reaching also important negative values and then it has some “wild jump” and returned to a zero value. Changing the damp parameter implies a change in the changing rate of pressure and in the rate of jumps.
So my question is: what is the meaning of this behaviour == am I choosing some unreasonable values of the damping parameter == there is a physical meaning to associate to this parameter? For a physical system like molecules in a solvent, I will say that the damping parameter is related to the viscosity of the solvent, but for condensed matter (what I am working on) the physical meaning of the damping parameter is less clear to me.
If you need some info about the simulations, just tell me.
Sorry for this long email, thank you in advance and have a nice day