A theoretical analysis of the lattice distortions induced by nonisovalent Ge2+, Sn2+, and Pb2+ substitutional impurities in crystalline NaCl, and of the off-center equilibrium position adopted by those impurities in their ground ns(2) electronic configuration is presented. The calculations are based in the cluster approach, and involve large active clusters embedded in an accurate quantal representation of the crystalline environment. The charge compensation problem is dealt with by considering several allocations of a cationic vacancy in the host lattice. The obtained distortions involve in all cases the concerted movement of a large number of host crystal ions. Those distortions are presented and discussed in terms of simple packing and Madelung considerations. (C) 2001 American Institute of Physics.