Calculation of depletion force

LAMMPS LAMMPS General Discussion
1

Hello everyone,
I am trying to calculate the depletion force between two arms placed in an active bath. For this, I have considered the depletion force as:

F(r, depletion) = f12(r) (forces between the two arms) + F1i(r) (forces from all the active particles in the bath).

To compute these forces, I used the following LAMMPS commands:

  1. compute force_gg arm1_poly group/group except_arm1
    and
    2 compute force_ft arm1_poly force/tally except_arm1

However, in both cases, I am getting a zero value, even though the depletion force should result in the arms pairing together.

Could you suggest a possible solution or point out any issues with the approach?

2

For compute group/group to produce forces and energies, three conditions have to be met:

  1. Both groups have to contain atoms
  2. The pair style has to be pair-wise additive or an EAM variant, i.e. contain a working Pair::single() function
  3. There have to be at least some pairs of atoms, always one of each group, that are have to be within the interaction cutoff

Confirming that all three conditions are met is your job.