Wide fluctuations of pressure in npt

Dear lammps users,

To optimize my system (a kind of metal organic framework), at first, I performed a CG simulation, after that a ramped NVT, in which the temperature increases from 0.1 to 298 K, was conducted and then a NPT simulation was performed. At the first two simulations, there is no problem with force fields and outputs like temperature, energy, etc, but in NPT simulation I do not get logical results, as, the fluctuations of the pressure, which is set to 1 atm, are from about -400 to 400 atmosphere! I used the following command for fix NPT with 1 timestep in real unit:

fix opt2 all npt temp {temp2} {temp2} 100 iso {press} {press} 1000

How can I fix this problem? Any help will be appreciated.

Regards,

Mina

Dear lammps users,

To optimize my system (a kind of metal organic framework), at first, I performed a CG simulation, after that a ramped NVT, in which the temperature increases from 0.1 to 298 K, was conducted and then a NPT simulation was performed. At the first two simulations, there is no problem with force fields and outputs like temperature, energy, etc, but in NPT simulation I do not get logical results, as, the fluctuations of the pressure, which is set to 1 atm, are from about -400 to 400 atmosphere! I used the following command for fix NPT with 1 timestep in real unit:

fix opt2 all npt temp \{temp2\} {temp2} 100 iso \{press\} {press} 1000

How can I fix this problem? Any help will be appreciated.

there is no convincing evidence, that there *is* a problem. pressure
is a macroscopic property. on the microscopic scale, it fluctuates. it
fluctuates quite a lot on systems, that are not very compressible,
like most condensed matter. like for temperature, the magnitude of
fluctuations correlates to the system size.

this is all old news and has been explained to people on this very
mailing list many times.

axel.