The ONIOM method was applied to study the hydrogen bond interactions between some CDK2 inhibitors and various models of the active site in CDK2/CyclinA system. It was found that according with the model's size, a good description of the molecular interactions inside the active site can be obtained. From best model, it was possible to obtain a reliable correlation between the total ONIOM energy and the biological activity reported for compounds studied. The results show that H-bond interaction energy is the principal component in this protein-ligand interaction and residues Lys89 and Asp86 are essential for great potency of compound NU6102. (C) 2009 Elsevier B.V. All rights reserved.