We describe the use of dipole lattice sums to estimate the electric field experienced by a molecule in a crystal, and apply it iteratively to ab initio calculations on a number of molecules of interest for nonlinear optical applications. The results demonstrate that it represents an efficient and reliable route to estimating molecular dipole moments in crystals. Typical dipole moment enhancements are in the range of 10-40%, agreeing well with the range resulting from partitioning of crystal Hartree-Fock electron densities, but much less than has been suggested in some previous work, and in particular from charge density analysis of X-ray diffraction data. (c) 2007 Elsevier B.V. All rights reserved.