We have carried out a thorough study of the methodology/basis set dependences in predicting first hyperpolarizability (beta) for polysilanenitrile (PSN) by using various wavefunction and density-based methods. Correlations between beta and the bond length alternation values (Delta r) are explained. Comparisons were made between PSN and the isoelectronic and isovalent analogues, and both the similarities and the differences in the beta value behavior in these three polymers are discussed by using the resonance structure theory. (c) 2005 Elsevier B.V. All rights reserved.