Pressure fluctuations in rigid/non-rigid system

Dear all,

I have a polymer system with both crystalline and amorphous system. Atoms in the crystalline regions are rigid. I want to maintain zero atm pressure in my system, but the total pressure wildly fluctuates between, 500 to -300 in real units. Below is my script,

fix 3 crystal rigid/nvt group 8 crystal1 crystal2 crystal6 crystal8 crystal11 crystal12 crystal13 crystal14 temp 300 300 100
fix 1 amorphous npt temp 300 300 100 iso 0 0 1000 dilate all
thermo_style custom step temp press etotal vol
thermo 100
timestep 0.05
run 100000
unfix 1
unfix 3

Since am trying to obtain mechanical response of this system, this pressure fluctuation will definitely affect my results.

Any suggestion to reduce the pressure fluctuations.

Dear all,

I have a polymer system with both crystalline and amorphous system. Atoms
in the crystalline regions are rigid. I want to maintain zero atm pressure
in my system, but the total pressure wildly fluctuates between, 500 to -300
in real units. Below is my script,

fix 3 crystal rigid/nvt group 8 crystal1 crystal2 crystal6 crystal8
crystal11 crystal12 crystal13 crystal14 temp 300 300 100
fix 1 amorphous npt temp 300 300 100 iso 0 0 1000 dilate all
thermo_style custom step temp press etotal vol
thermo 100
timestep 0.05
run 100000
unfix 1
unfix 3

Since am trying to obtain mechanical response of this system, this
pressure fluctuation will definitely affect my results.

Any suggestion to reduce the pressure fluctuations.

​the topic of pressure fluctuations is rather frequently discussed on this
mailing list, so you may want to search the mailing list archives for more
detailed discussions. a look into a statistical mechanics text book is
warranted as well.
in short, there are ​three major points:

- condensed system are not very compressible, so large *local* fluctuations
of the virial stress are to be expected. you have fluctuations for the
kinetic energy just as well, but they just are not of the same relative
size compared to the typical values.
- as the fluctuations are local, they become smaller with a larger system.
this holds true for both temperature and pressure (virial stress).
- for a system in equilibrium, averaging over time is equivalent to
averaging over a larger sample.

thus you need to set up a larger system and/or average over time to reduce
the fluctuations.

axel.