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[lammps-users] Harmonic wall to control external pressure in non-periodic direction
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[lammps-users] Harmonic wall to control external pressure in non-periodic direction

From: "ONOFRIO, Nicolas [AP]" <nicolas.onofrio@...6977...>
Date: Fri, 22 Dec 2017 02:57:20 +0000

Dear LAMMPS users,

I want to follow-up on a question that was discussed on the forum:
I would like to control the pressure on a non-periodic dimension using a moving harmonic wall, as suggested by Steve in the above discussion. 
The “barostat” works for some time and then becomes chaotic and crash.

I made a toy system made of water molecules periodic in y, z but not in x where I put a harmonic wall at the position defined by the following equation (I consider only the xlo side here):
x = x0 + v*t

The velocity of the wall is a function of time and adjusted every cycle from the velocity at the previous cycle (with a loop):
v(i) = v(i-1) + gamma*dv

Gamma is given by: 
gamma = (Pext - P)/Pext

with Pext the external pressure and P the actual wall pressure computed from the force on the wall via: P = 68568.415*abs(fwall)/area (in real units) and fwall is time-averaged over 10 steps corresponding to one cycle.

Everything work as expected at the beginning and the pressure equilibrates well however after ~500 cycles, the pressure starts to diverge and rapidly becomes chaotic and the simulation crash.
I would appreciate any advice to solve this.
I know there are other simulation options but I just would like to understand this behavior.
I attach a plot of the actual pressure P (Atm) as a function of cycles (10 MD steps) with Pext = 1000 Atm as well as the corresponding input, force field and data files.

I use REAX/C & LAMMPS-17Nov16.

Thank you and regards,

Nicolas Onofrio


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