Reax/c doesn’t like NULL in its pair_coeff command. Create a dummy atom in forcefield file and replace NULL with dummy atom type. If that doesn’t work, you just have unsuitable parameters for your system.
Ray
there is another inconsistency in the input deck here.
the charge equilibration is applied to all atoms, and the parameters are supposed to be taken from the reax/c pair style.
so how would the reax/c pair style know what to do apply for your argon atoms, and should QeQ be applied to them at all, considering, that you are not using reaxff to describe them?