What's the difference between these two lines?

LAMMPS LAMMPS General Discussion
1

Common lines:
region efieldarea block INF INF INF INF 44.0 49.0
group efieldapply region efieldarea

Command 1:
fix 3 all efield 0.0 0.0 0.3 region efieldarea

Command 2:
fix 3 efieldapply efield 0.0 0.0 0.3 region efieldarea

I want only the atoms that are in the region real time experience the external force and not the other atoms. For example a new atom entering the region should experience the force.

2

This should be explained in the documentation for the fix command and the fix efield command.

3

If I can reframe my question. What would be the best way to achieve my goal?

4

This is not a question about “best”, but a question of “how”.
And as I already pointed out, the answer is in the two documentation pages I mentioned. It is pretty straightforward to address this with just logically applying what the documentation says.

Please note that it is the intention of the majority of the people responding to forum posts to educate users to become better at using and understanding LAMMPS on their own and not to just solve their problems for them. We notice that if we do the latter, people will stop trying to solve problems on their own and just rely on us to do what is effectively their job.

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