Intermolecular transfer integrals, and associated band-structures of organic molecular materials can be calculated through a dimer approach. Extensive numerical studies are performed on an ethylene pi-dimer to investigate the basis sets dependence of transfer integrals. Convergence of calculated transfer integrals is achieved with respect to both Gaussian and plane-wave basis sets, provided the same level of theory is used. Effects of diffuse and polarization Gaussian functions on transfer integrals are identified. Comparison of experimental and theoretical values of transfer integrals of the TTF-TCNQ charge transfer salt is also presented. (C) 2004 Elsevier B.V. All rights reserved.