The molecular dynamics method combined with a quantum mechanical calculation was used to simulate the collision of an argon atom with an argon cluster ion, Ar-n(+) (n=3-23), containing a given amount of internal energy. Two processes were observed; (i) evaporation caused by collisional energy transfer to the internal degrees of freedom vs (ii) fusion of the target atom with the cluster ion via complex formation. The total reaction cross sections were compared with those experimentally obtained. It is found that the branching fractions of the evaporation and the fusion depend critically on the impact parameter.