The authors present a method to calc. NMR chem. shielding tensors in condensed phases by a hybrid quantum mech./mol. mech. (QM/MM) approach. The authors propose a modification of the conventional QM/MM technique, adding a general repulsive potential to the electronic interaction Hamiltonian. This universal potential is motivated by the absence of Pauli repulsion in std. interaction potentials that are based only on classical point charges. The authors apply the method to realistic systems composed of mols. with strong dipolar character, thus forming strong hydrogen bond networks. In particular, the authors present calcns. for liq. water and a proton conducting org. crystal. The elec. field and direct contact effects of surrounding mols. play a crucial role in the NMR resonance lines of such materials. The results are in very good agreement with full quantum calcns. as well as with expt. Thus, this new combination of ab initio NMR chem. shift calcns. with a QM/MM modeling of extended systems provides an improved tool for the anal. of complex biol. and chem. systems, such as polymers and proteins. [on SciFinder (R)]