Hi Mine,
Your results are probably correct, if you're just looking at the energies in some well-ordered initial configuration. Two things are useful to know here. First, it's not really the value of nn2 that controls whether you get second neighbor influence, it's Cmin. If Cmin(1,1,1) is below a certain value -- I think around 1.0 for FCC, but it's not hard to work out the correct value -- then second neighbors are completely screened and have no effect if you're in the reference configuration.
Second, the nn2 parameter has the effect of modifying the pair potential so that you get the correct energy per atom in the reference configuration (the "Ec" value you set). In fact, what the nn2 parameter does is to make sure that the energy comes out correct. So, if the value of Cmin is below the threshold value, then nn2 should be set equal to 1, otherwise the energy per atom in the reference state will come out wrong.
Since you're seeing no difference when you change the value of nn2, I'm guessing that Cmin is above 1.0 (the default is 2.0 if you're not setting anything).
Hope this all made sense.
Regards,
Greg