Using a real-space ab initio pseudopotential method, we examine the electronic properties of Li interstitial donors in ZnO nanocrystals. We find significant charge redistribution of the donor electron around the Li interstitial atom in ZnO nanocrystal. As a result, the recipe of Van de Walle and Blochl frequently used to calculate hyperfine splitting using pseudopotentials is no longer applicable. We generalized their recipe to situations where the off-site contributions of the donor charge density cannot be neglected. Our calculated hyperfine splittings agree well with recent experimental measurements and exhibit a strong size dependence. (c) 2008 Elsevier B.V. All rights reserved.