Dear Steve and Ian,
Concerning the interaction force between Al bulk and Si indenter, if you conduct the indentation simulation with “fix indent” command, you can get that force via commands as follows:
fix_modify ID energy yes; thermo_style custom step f_ID[1/2/3]. The ID is the ID for the fix indent.
However, Steve, I also have a question about that. I get that force successfully in that way when the indenter penetrates into the bulk material, but the three force vectors are all zero when the indenter withdrawed from the bulk material. Do you have such experience?