# [lammps-users] velocity command

Dear lammps-user ,

I have a system of two rods with a bunch of water molecules around, I initialized the system in a way that there is no overlap between rod ions and water molecules. But I need to change the distance between the rods, I can initialize the system each time for each value of rod separation but it is time consuming. I tried to use displace command but it seems that for some cases the rodions gets so much close to water molecules in a way that the strong repulsion cause missing of some the ions. So I am trying to do something different. I want to change the distance between two rods using the velocity command, in this way lammps change the distance between two rods slowly (considering small velocity in one direction) such that the water molecules have the opportunity to move in a way that there will be no collisions. So I need to give the rodions small velocity for a short time and then stop them in order to reach the certain desired distance. But I donâ€™t know how to do it, It would be great if you can help with this problem.

Best,

Dear lammps-user ,

I have a system of two rods with a bunch of water molecules around, I
initialized the system in a way that there is no overlap between rod ions
and water molecules. But I need to change the distance between the rods, I
can initialize the system each time for each value of rod separation but it
is time consuming. I tried to use displace command but it seems that for
some cases the rodions gets so much close to water molecules in a way that
the strong repulsion cause missing of some the ions. So I am trying to do
something different. I want to change the distance between two rods using
the velocity command, in this way lammps change the distance between two
rods slowly (considering small velocity in one direction) such that the
water molecules have the opportunity to move in a way that there will be no
collisions. So I need to give the rodions small velocity for a short time
and then stop them in order to reach the certain desired distance. But I
don't know how to do it, It would be great if you can help with this
problem.

just set the velocity to a small enough value that you don't cause a
shockwave and then use fix setforce with 0.0 0.0 0.0 on the rods.
following newton's first law your rods will move with constant velocity
and you can easily compute how many time steps you need to make
them move a given distance.

an alternate option would be to use fix spring to drag the rods to
some defined coordinate.

axel.