Conjugated polymers (CPs) have been used as highly responsive optical sensors for chemical and biological targets. The density functional theory (DFT), the time-dependent density functional theory (TDDFT) and the singly excited configuration interaction (CIS) approach were employed to simulate the absorption and emission spectra of a model monomer unit of the typical CP, poly[9,9'-bis-(60-N, N, N-trimethylammonium) hexyl]fluorene-alt-4,7-(2,1,3-benzothiadiazole)] (PFBT). Various basis sets were applied in the calculations and their effects on the predictions for the optical properties of FBT are discussed. The results suggest that the TD-B3LYP method with basis sets larger than 6-311G(d, p) provides a suitable approach for investigations of the studied system. (c) 2008 Elsevier B. V. All rights reserved.