[lammps-users] Question about fix setforce

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1

Hi all: I am using fix setforce to zero out z component of the force
on individual atoms within one group. This fix computes the total
force on the group of atoms before the z components are set zero by
the fix. I dumped the total force. But it seems to me that the total
force has been normalized by the number of all the atoms in my system,
as it is extremely small. Does LAMMPS do this normalization?

Thank you.

Best,
Ting

2

From the fix setforce doc page:

This fix computes a global 3-vector of forces, which can be accessed
by various output commands. This is the total force on the group of
atoms before the forces on individual atoms are changed by the fix.
The vector values calculated by this fix are "extensive".

So if you are running in LJ units, the thermo output will
normalize all extensive quantities by default, unless you
use thermo_modify norm no.

Steve

3

Thank you.

Best,
Ting