spring constant in fix smd cfor mode

Hi,

I have a question about the fix smd constant force (cfor) mode command in lammps, which can be used for applying a constant force to a group of atoms with a spring: https://lammps.sandia.gov/doc/fix_smd.html

In contrast to the constant-velocity (cvel) mode of fix smd command which asks for a spring constant as one of its input, the constant-force (cfor) mode doesn’t ask for a spring constant and only requires the value of constant force in its input. May I ask if there is no spring constant used in the fix smd cfor mode, or the value of spring constant should be inferred from the chosen value of constant force and the distance between the center of mass of group (of atoms) and the tether point (using the restoring force equation F=K(R - R0)Mi/M ?), at the instance of initiation of the fix? Alternatively, if none of these is the case, is there any other way to provide the value of spring constant for fix smd cfor?

I am using lammps version July 21, 2020.

Thank you.

Behrooz

Hi,

I have a question about the fix smd constant force (cfor) mode command in lammps, which can be used for applying a constant force to a group of atoms with a spring: https://lammps.sandia.gov/doc/fix_smd.html

In contrast to the constant-velocity (cvel) mode of fix smd command which asks for a spring constant as one of its input, the constant-force (cfor) mode doesn’t ask for a spring constant and only requires the value of constant force in its input. May I ask if there is no spring constant used in the fix smd cfor mode, or the value of spring constant should be inferred from the chosen value of constant force and the distance between the center of mass of group (of atoms) and the tether point (using the restoring force equation F=K(R - R0)Mi/M ?), at the instance of initiation of the fix? Alternatively, if none of these is the case, is there any other way to provide the value of spring constant for fix smd cfor?

you don’t need a spring constant, because you are applying a constant force.

axel.