# force extraction

Dear All,

I have applied a “fix move” rate to one end of CNT while keeping the other end fixed (using fix setforce 0 0 0). I would like to know what is external force that applies on the end. The force to me is the total force in axial direction on the end. But I do not know which command or command combination I should use. Thank you.

Regards,
Mengyu

Dear All,

I have applied a "fix move" rate to one end of CNT while keeping the other
end fixed (using fix setforce 0 0 0). I would like to know what is external
force that applies on the end. The force to me is the total force in axial
direction on the end. But I do not know which command or command combination
I should use. Thank you.

fix move does not apply any force. it just moves the atoms.

axel.

Thank you Axel.

But I would like to know what force will apply on the end from other atoms. Imagine you do not have this “fix move” rate, then atoms will go back to its original positions.

Thank you Axel.

But I would like to know what force will apply on the end from other atoms.
Imagine you do not have this "fix move" rate, then atoms will go back to its
original positions.

try using compute reduce then.

axel.

I guess you are suggesting using

compute stress/atom + compute reduce

I do not think this should work as computer stress/atom has a unites of pressurevolume (= forcedistance). But I only want to know force each atom has and want to sum them together at the end. I do not know how to define an appropriate volume for an atom.

I guess you are suggesting using

compute stress/atom + compute reduce

no. i am not.
just use plain compute reduce.

I do not think this should work as computer stress/atom has a unites of

i didn't say anything about stress.

pressure*volume (= force*distance). But I only want to know force each atom
has and want to sum them together at the end. I do not know how to define an
appropriate volume for an atom.

if you read the documentation for
compute reduce, you'd have seen
force components.

axel.

Axel means to use compute reduce to directly sum the
force on each atom in the end you are interested in.

You might also think about checking the force on
the other end, the one you are using fix setforce 0.0
on. That fix stores the force on the atoms before
they are zeroed, so it can be printed. By symmetry
it may be equal to the force on the other end.

Steve