[lammps-users] outputting force from a frozen atom?

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1

Hello all,

This is similar to a question that was asked a few days ago, but is
there a way to freeze an atom (setforce group 0.0 0.0 0.0) and be able
to output the force _before_ the setforce fix clears it to zero? For
doing atomistic mechanical testing, this would be necessary for
determining how much force is needed to hold a molecule in place as it
is moved through a system over time. (run x every 1000
displace_atoms ...) Any ideas on how to accomplish this?

Thanks,

Dusty Majure

2

The fix setforce command stores the force on the
entire group of atoms before it changes it. This
can be accessed by thermo output or a variable, etc.
So if your group was the single atom you wanted,
it would be what you ask.

Steve

3

Steve,

Thanks for pointing me in the right direction, but I'm still having
problems getting it to work after reading the documentation. Below is a
snippet of my input. I am wanting thermo to output the total force (or
at least all the forces on the atoms) of the groups one_ring and
outer_rings in the Z-direction. Could you tell me where I went wrong?
All I get from the thermo output for my variables are 0.0's.

# Get the total z-force for both groups
variable z1 equal fcm(one_ring,z)
variable z2 equal fcm(outer_rings,z)

# Assign fix to hold these rings in place on z-axis
fix 1 one_ring setforce NULL NULL 0.0
fix 2 outer_rings setforce 0.0 0.0 0.0

# Change thermo to output my variables
thermo 4000
thermo_style custom step cpu ke pe etotal v_z1 v_z2

# Run simulation
run 1200000 every 4000 displace_atoms center move 0.000 0.000 0.05
units box

Thank you,

Dusty

4

It's the fix that stores the force before it was changed. So you
want somthing like:

thermo_style custom step cpu ke pe etotal f_1[3] f_2[3]

for z-component of force.

Steve