I’m simulating water (SPC/E) with two parallel rigid stationary walls immersed in it under NPT conditions
The relevant portion of the script is:
group walls type 3 4
group walla type 3
group wallb type 4
group water type 1 2
compute cmpTemp water temp
compute cmpPress all pressure cmpTemp
fix fShake water shake 0.0001 20 0 b 1 a 1
fix fRigid walls rigid/nve group 2 walla wallb force * off off off torque * off off off
fix fNPT water npt temp 298.0 298.0 100.0 iso 1.0 1.0 1000 dilate water
fix fNPT water npt temp 298.0 298.0 100.0 y 1.0 1.0 1000 z 1.0 1.0 1000 couple yz dilate water
fix_modify fNPT temp cmpTemp press cmpPress
Above there are two possible versions of “fix npt.”
If I use the first one (isotropic), then the x-coordinates of each wall changes over the course of the simulation. Specifically, each wall atom in a given wall moves by the same change in x at each timestep. In addition, their relative position is not preserved.
If I use the second version, then the positions of the walls remain constant.
I was wondering why this difference existed, as I would have expected that position of the walls would not change in the first version of “fix npt.”
Any help understanding this discrepancy is greatly appreciated.