The C-s symmetry reaction of the H-2 molecule on a Pt-4 (111) clusters, has been studied using ab initio multiconfiguration self-consistent field plus extensive multireference configuration interaction variational and perturbative calculations. The H-2 interaction by the vertex and by the base of a tetrahedral Pt-4 cluster were studied in ground and excited triplet and singlet states (closed and open shells), where the reaction curves are obtained through many avoided crossings. The Pt-4 cluster captures and activates the hydrogen molecule; it shows a similar behavior compared with other Pt-n (n=1,2,3) systems. The Pt-4 cluster in their lowest five open and closed shell electronic states: B-3(2), B-1(2), (1)A(1) (3)A(1), (1)A(1), respectively, may capture and dissociate the H-2 molecule without activation barriers for the hydrogen molecule vertex approach. For the threefolded site reaction, i.e., by the base, the situation is different, the hydrogen adsorption presents some barriers. The potential energy minima occur outside and inside the cluster, with strong activation of the H-H bond. In all cases studied, the Pt-4 cluster does not absorb the hydrogen molecule. (C) 2004 American Institute of Physics.