[lammps-users] about "fix npt" and "fix thermal/conductivity"

hi, everyone

i meet two questions when using lammps.

The first is about fix npt command. At beginning , temperature and pressure values always oscillated violently, both seem to never converge, temperature ,howerver, ocillated much more gently than pressure. As lammps user mannual indicat, i adjust drag keywor value from 2 to 200, pressure ocillated just a little gently. I also try to use other commands combination ,e.g. fix press/berendsen and fix nvt,
but lammps always gives “Nan” in output. What can i do to get desirable pressure and temperature?

The second is about fix thermal/conductivity command. I try this command more than nearly 100 times, no matter what are Nevery and Swarp, the temperature is either none linear or layer 1 is cold than middle layer. By observing carefully ,i found there are no tempperature changes in almost every layer except lay1 and swapping layer from beginning to end. It seems in those layers no response occure or no dynamic iteration occure at all, so all particales in those layers cannot change their average temperatures. What is the reason?

Any help would be appreciated.

wang bochun

Re: fix npt and thermostat. I would always run your system initially
with fix nve and see if it is more or less stable. If it is not, then
nvt or npt
are not going to magically going to equilibrate your system instantly.
Common problems with npt and nvt are using bad damping constants.
You can also try other thermo/barostat options. There are many of
them in LAMMPS.

Re: fix thermal/cond - I know of no bugs with this command. I'd try
setting up a temperature profile in a simple 2d liquid or solid first.

Steve