Dear Dr. Axel Kohlmeyer and LAMMPS Users,
Hello All. I am currently equilibrating a 2D polymer network subject to NPT ensemble. I would like to obtain a stress free state of this 2D network prior to conducting a stress strain test.
I first used the soft cosine potential that govern the interaction between non-connected particles. The connecting particles that make up the chains are governed by the FENE potential.
I first ran a NVE ensemble and then switched to NPT to relax the 2D system of polymer network and find its equilibrated stress free state.
Using LJ units, I conducted the NPT ensemble at T = 1.0 and p = 0.0. (This is the NPT ensemble that I use to equilibrate my 3D polymer network)
However, the volume (2D area) of the 2D polymer network keeps inflating although the pressure reaches p = 0.0 and T = 1.0. (Interestingly, when I use 3D polymer network, I do not have this problem as the polymer equilibrates to its melt density using Kremer-Grest parameters under NPT: p = 0.0 and T = 1.0)
I am not understanding why the 2D volume is inflating when, in fact, the pressure (e.g pxx and pyy) reaches steadily at 0.0 thereby nothing to push the boundaries of the 2D region outward. Moreover, I see that the free energy of the system remains constant as I observe the total potential energy remains steady.
I searched the previous mailing list, and I see that bad initial geometry (overlapping beads) or not running NVE ensemble may cause this volume expansion. However, I am using soft cosine potential which prevents any sudden repulsion due to overlapping particles.
I would greatly appreciate if I could request for an advice that determines the cause for the continuous volume expansion.
I have attached a small simulation that replicates this issue of this 2D volume expansion under NPT.
Thank you so much for your time.
Sincerely,
Masato Koizumi
2D_NPT_Equilibration.in (871 Bytes)
2D_Polymer.dat (533 Bytes)