In the present study, we report on the results of calculations of non-linear optical properties (NLO) of the simplest pyridinium N-phenolate betaine dye [4-(1-pyridinium-1-yl)phenolate]. The two contributions to the first-order hyperpolarizability (beta), namely vibrational (beta(v)) and electronic (beta(e)) were calculated at the Hartree-Fock (HF) level as well as using second-order Moller-Plesset (MP2) perturbation theory. The evaluation of both contributions to beta using the MP2 method brought an interesting and unusual observation: Contrary to other pi-conjugated molecules, the inclusion of electron correlation diminished the values of (beta(e)) for the investigated betaine dye. The effect upon including electron correlation through second-order in perturbation treatment leads to the change of sign of vibrational counterpart of the first-order NLO response of the molecule under study. (c) 2005 Elsevier B.V. All rights reserved.