The interactions of the PO4- groups of DNA and the positive parts of lysine/arginine were calculated in the nucleosomes using the ONIOM method. For the model system (3H(2)O, K+ ion, PO4- group, positive side chains of lysine, arginine, resp., Cl-) a triple-zeta basis set with polarization functions was applied both at the HF and MP2 levels. For the real system two dezoxyriboses and two guanines were added, the calculations were performed with a double-zeta basis set at the HF level. The direct PO4-center dot center dot center dot Lys(+)/Arg(+) bonds are similar to 5 eV strong. These decrease by similar to 4 eV due to the Cl- ions. The water-mediated PO4-center dot center dot center dot H2O center dot center dot center dot Lys(+)/Arg(+) bonds are similar to 1 eV strong and the Cl--s hardly change their strength. (C) 2012 Elsevier B.V. All rights reserved.