The third-order resonant and static nonlinear optical polarizabilities of the donor-donor and donor-acceptor substituted pi conjugated molecules are calculated using the third-order response formalism in combination with time-dependent Hartree-Fock (TD-HF) and density functional theory (TD-DFT) methods. Performance of different levels of theory for excited state structure and nonlinear optical responses has been analyzed. Since the exact computations are fairly expensive, and only a few components of the cubic polarizability (corresponding to the Liouville space paths) are important, numerically efficient approximations are suggested. (C) 2004 Elsevier B.V. All rights reserved.