Hartree-Fock calculations are reported for the effect of donor dopant concentration on the static longitudinal second hyperpolarizability of individual polyacetylene chains using explicit alkali (Li, Na, K) atoms and varying chain lengths. Enhancement factors approaching two orders of magnitude, with respect to the undoped chain, are found for potassium at the largest concentrations that can be sustained without dissociation. Sample MP2 calculations indicate that the Hartree-Fock structure-property relations are preserved. (c) 2005 Elsevier B.V. All rights reserved.