Quasiclassical trajectory calculations were carried out to investigate the one-atom cage effect in I 2 Ar(B) van der Waals complex. To compare with a recent exact wave packet calculation, the complex is restricted to a collinear configuration and the same potential energy surfaces were used. Very good agreement with the exact results demonstrates the applicability of the quasiclassical method to this problem. Without invoking any non-adiabatic mechanism, our theoretical results agree well with experimental observations at all wavelengths considered. This work demonstrates the importance of mechanical impact mechanism and the insignificance of a non-adiabatic mechanism.