In PairAIREBO::bondorderLJ correct omega sum d/drij.

The code tries to make this distinction between the real distance (r23) and the facticious one (rij), but does not do so very well.
It is better if those two variables have the same value everywhere, and apply the correction where necessary.
The current way to use the values is incorrrect.

Remove those calculations that effectively are derivatives w.r.t. |rij| (the facticious distance), is constant and thus the chained derivative (d|rij|/dRij) is always zero.

Apply the corrections due to drij/dRij in the sum omega term.