Volume Expansion using fix NVT and fix GCMC

Dear Dr. Stefan Paquay and LAMMPS Users,

Hello Dr. Paquay. Thank you so much for your e-mail.

I currently have a simulation box that contains 100 polymer chains each consisting of 100 monomers (coarse-grained beads). The polymer chains are equilibrated and crosslinked where the connected monomers and the non-connected monomers are governed by the FENE potential and LJ potential, respectively.

I would like to have this polymer matrix swell water molecules by using fix gcmc. Fix gcmc will keep the chemical potential and the temperature of the polymer matrix constant by specifying the user-input mu and the T. However, the volume of the polymer matrix will inflate since water molecules will be entering the polymer matrix. I would like to have the swelling behavior (volume inflation) stop when the chemical equilibrium is reached. The chemical potential at chemical equilibrium is the mu that I will specify.

When I simulate my swelling using fix nvt with fix gcmc which results in muVT ensemble, I noticed that the volume of the simulation box remains constant even though the water molecules are being added into the polymer matrix when in fact the volume should inflate as if it is swelling.

I understand that fix nvt does not enforce constant volume and adjusts the temperature and performs the time integration via Nose-Hoover. However, in my case, the constant volume is enforced when water molecules are being inserted via fix gcmc.

I was curious if there is a way to have the volume of the simulation box change and eventually stop when the chemical equilibrium is reached (specified by user-input mu).

Once again, thank you so much for your time in providing me with valuable advice. I have displayed a portion of my input script in case if needed.


Masato Koizumi

units lj
atom_style full
neighbor 0.36 bin
neigh_modify delay 2
pair_style lj/cut 1.122462
bond_style fene
special_bonds lj 0.0 1.0 1.0
read_data DataFile extra/atom/types 40 extra/bond/types 40
mass * 1.0
pair_coeff * * 1.0 1.0 2.5
bond_coeff * 30.0 1.5 1.0 1.0
pair_modify shift yes
timestep 0.0010
reset_timestep 0

reset_timestep 0

group addatoms type 2
region mobile block EDGE EDGE EDGE EDGE EDGE EDGE side in
group mobile region mobile
molecule dimer molecule.dimer

fix 7 all nvt temp 1.0 1.0 2.0
fix 8 addatoms gcmc 10 100 100 0 3456543 1.0 -2.5 0.1 region mobile mol dimer

delete_atoms overlap 0.0 addatoms all

run 10000