[lammps-users] maintaining density

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1

Dear all,

I have a question about equilibrating a box of polymeric chains. I prepare a geometry for simulation with the experimental density of the polymer. So the box has enough molecules to mimic the density at room temperature and atmospheric pressure. However, when I start equilibrating after minimization, I find that the pressure values are ~10000 atmospheres when equilibrating with fix-NVE+langevin. Subsequently I apply NPT to bring the pressure to 1 atm, and after the equilibration of pressure, temperature and energy, the box seems to have slightly expanded in all the dimensions, which essentially reduces the density. Now I am unsure whether to use high values of pressure to maintain the original density or use a lower density at atmospheric pressure. Is there a different approach to obtain the correct density at equilibrium and room temp+atm conditions or does the reported value of high pressure indicate something else?

Thank you
Ganesh

2

Dear all,

[...]

pressure to 1 atm, and after the equilibration of pressure, temperature and
energy, the box seems to have slightly expanded in all the dimensions, which
essentially reduces the density. Now I am unsure whether to use high values
of pressure to maintain the original density or use a lower density at
atmospheric pressure. Is there a different approach to obtain the correct
density at equilibrium and room temp+atm conditions or does the reported
value of high pressure indicate something else?

this could indicate one of two things:
1) your system is not fully equilibrated yet.
2) the parameters that you are using are not good
    for the system you are trying to simulate.

cheers,
      axel.

3

Re: Axel's point (2) - the force field params are key to
getting good pressures. Also things like cutoffs, long-range
corrects to the pressure.

Steve