AbstractThe CuPt3 cluster with H2 interaction was studied by means of Hartree–Fock self‐consistent field (HF‐SCF) and multiconfigurational SCF plus variational and multireference second‐order Möller–Plesset perturbational configuration interaction (CIPSI) calculations, using relativistic core potentials. The CuPt3‐H2 interaction was carried out in Cs symmetry for both: the H2 in the Cs reflecting plane and bisected by the Cs reflecting plane. The valence electronic configuration and energy separation of the three low‐lying states of CuPt3 tetrahedral pyramid cluster was determined: 2A″, 2A′, and 4A′ electronic states. We found that the (2A″) ground state captures and dissociates H2 without energy barriers only by a Pt vertex of the CuPt3 cluster. Also, the CuPt3 cluster, in its (2A′) first excited state, by the Cu or Pt center can capture and dissociate the H2 molecule. Meanwhile, the same state for the CuPt2 and Pt3 face can capture and break H2. We have found that CuPt3 cluster in its (4A′) second excited state spontaneously activates and breaks the hydrogen molecule, with some exception, when H2 is approaching to a copper vertex. In this work we found that the CuPt3‐H2 and Pt4‐H2 interactions show similar capture and activation of H2 behaviors such as depth of wells, barriers, adsorption distances, and HH bond relaxation. Even in the case of a single Pt atom substitution by a Cu in a Pt4 cluster, there is no loss of the H2 activity compared with pure metal cluster. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002