In my system, a freezing wall is successfully built in the water by the commond “fix wall set force 0.0 0.0 0.0”. Therefor, there is no force acting on these wall atoms. However,I want to dump the actual force exerting on the fixed wall (from the solvent). Is there a simple way to obtain it ? If not, Can someone tell me how to modify the source procedure?

Define a computation that calculates the total energy and force interaction between two groups of atoms: the compute group and the specified group2. The two groups can be the same. The interaction energy is defined as the pairwise energy between all pairs of atoms where one atom in the pair is in the first group and the other is in the second group. Likewise, the interaction force calculated by this compute is the force on the compute group atoms due to pairwise interactions with atoms in the specified group2.

The energy and force are calculated by looping over a neighbor list of pairwise interactions. Thus it can be inefficient to compute this quantity too frequently.

The fix setforce command stores the force you are asking and
you can output it with thermodynamics (or use it in other parts
of LAMMPS, like from a variable). See the doc page
for fix setforce and thermo_style.