[lammps-users] Aveforce: Doubt regarding how the aveforce works

Hello,

I have a fluid layer confined between 2 rigid walls called lower and
upper. An upward force is applied on the lower wall with the following
commands.

fix 4 lower setforce 0.0 0.0 NULL
fix 5 lower aveforce 0.0 0.0 0.10
fix 6 upper setforce 0.0 0.0 0.0
fix 7 lower rigid molecule

I observe that that the wall on which the force is applied oscillates
for some time before it comes to a stationary state. Is this
oscillations an artifact of the MD simulation? Is that the way how
aveforce works? Putting in another way, could the oscillations be a
result of the way in which constant load is maintained in the
simulation rather than a physical response of the confined liquid
itself? Any suggestions or comments is greatly appreciated.

Thanking You

With Regards
Ajay

I think oscillations are happenin due to the fact that your system is not equilibrated prior to applying set aveforce. If that is the case, please equilibrate it first with just using setforce and then use aveforce when your system is equilibrated.

Vikas.

To add to Vikas's comment. if you push hard on a springy material,
then the wall may oscillate. If you are not well equilibrated, then
you may have large wall displacements, and this effect will
be enhanced.

Steve