Inquiry on the Difference Between Fix press/berendsen and Fix NPT

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1

Dear Dr. Axel Kohlmeyer and LAMMPS Users,

Hello All. I am simulating an MD run that relaxes a system of equilibrated polymer chains under constant temperature and the pressure and the volume is allowed to fluctuate.

The nonconnected beads in a chain are governed by repulsive LJ potential and connecting bonds are governed by the FENE potential as displayed in the following:

pair_style lj/cut 1.122462

bond_style fene
special_bonds lj 0.0 1.0 1.0

pair_coeff * * 1.0 1.0 1.122462
bond_coeff * 30.0 1.5 1.0 1.0
pair_modify shift yes

What is the difference between the following two cases that does stress relaxation of the polymer chains

Case 1:

fix 1 all langevin 1.0 1.0 2.0 542305
fix 2 all press/berendsen iso 0.0 0.0 10.0
fix 3 all nve
run 1000000

Case 2

fix 1 all npt temp 1.0 1.0 2.0 iso 0.0 0.0 10.0
run 1000000

I ran both of these MD simulations without any error and I plotted the volume of the simulation box with respect to time. I see that the Case 2 resulted in a slightly higher volume at the end of the run (meaning that the relaxation inflated the system) but overall I do not see any significant difference between these two cases.

I would greatly appreciate if anyone could provide me with any comments. Thank you so much for your time.

Sincerely,

Masato Koizumi

2

This question is slightly outside of the scope of this mailing list because it’s really about the underlying methods rather than about how to use lammps. You really should read the literature on the methods behind these lammps commands. Very briefly:

Nosé-Hoover and Langevin are different equations of motion and should give similar averages of statistical quantities but different trajectories. Nosé-Hoover can have issues covering phase space in some pathological cases.

Using a Berendsen batostat means your results will not properly sample the isothermal-isobaric ensemble, hence it should not be used for the actual acquisition of data. A Nosé-Hoover barostat should be fine.