The present study investigates the potential of the Largest Lyapunov Exponent (LLE) for the quantification of AF complexity as a marker of antitachycardia pacing (ATP) effectiveness in a biophysical model of the human atria. From ongoing simulated atrial fibrillation, 20 transmembrane potential maps were used as initial conditions for a rapid pacing from the septum area (at pacing cycle length as a percentage of the AF cycle length). The LLE was separately computed during AF and ATP for the transmembrane potential time series recorded at a single site in the right atrial posterior wall. The averaged results over all 20 simulations show that the LLE decreases during ATP relative to AF. These results suggest that LLE may serve as an indicator of AF complexity and also as a discriminating metric in automatic assessment of AF capture during ATP.