computing addforce

dear all
I saw a strenge thing!
i used "fix addforce fx 0 0 " to a group of atoms.then i used “compute reduce ave and sum” to calculate the average and sum force to that group respectively.
suppose i have 100 atoms in my group.
when my style was “real”. fx(sum) was 100multiply fx(ave).
But when my style was LJ fx(sum) was equal to fx(ave).
what is the problem?

If you mean you defined a variable named fx, then
the correct syntax is
fix addforce v_fx 0 0

See the doc page for fix addforce and its examples.