Hi, all. I have a question about the relative merits of different thermostatting protocols (available within LAMMPS) at low T. I will be running NPT-ensemble simulations, so obviously the Nose-Hoover thermostat/barostat combination seems a natural choice. One problem that comes to mind (and is mentioned in the docs), however, is that the N-H thermostat can produce “ringing” in sims. of crystals (and I am simulating crystals, so this is a worry). One way around this is to use a thermostat ‘period’ tau_{therm} which is well below (and not a divisor of) the barostat period tau_{bar} (i. e. the time over which P is brought to P_target).
I guess another way is to use the ‘drag-damping’ functionality available in fix_npt. The webpage says “A value of 0.0 (no drag) leaves the Nose/Hoover formalism unchanged. A non-zero value adds a drag term; the larger the value specified, the greater the damping effect. Performing a short run and monitoring the pressure and temperature is the best way to determine if the drag term is working. Typically a value between 0.2 to 2.0 is sufficient to damp oscillations after a few periods.”
However, the drag may itself affect the thermodynamics, in ways I don’t have a good feel for. My question, then, is:
Is anyone here expert in sims. in this regime and familiar with the ‘pitfalls’? Or at least, could someone point me towards article(s) discussing them? Both NPT-at-low-T and how adding drag affects things? This is a bit above the level of say, the Frenkel & Smit textbook.
For example, I’ve heard the ringing is worst at ‘low’ T, but what is ‘low’? 10% of the melting temperature? Specific to LAMMPS, is it better (in terms of physical realism) to increase or decrease the drag as T increases, or leave it constant?
I guess a one-sentence summary is (ideally) I’m looking for an advanced thermostatting reference that covers the ‘NPT-at-low-T’ options within LAMMPS, and any help would be appreciated.
Thanks,
Rob