With the intention of determining the effects of core structures and alkyl chain lengths, the geometric and electronic structures of five sets of cyano-containing mesogen molecules with different rigid cores and variable alkyl chain lengths from 1 to 10 carbon atoms have been investigated theoretically by quantum chemical methods. In order to determine the most suitable quantum chemical method and the basis set, geometry optimizations of the most stable conformers have been performed with the semiempirical AM1 and PM3 methods, ab initio HF/3-21G and HF/6-31G* and DFT/B3LYP/6-31G*. The equilibrium geometric structures, dipole moments and charge distributions of the mesogen molecules have been calculated. The reliability of the methods used has been assessed by comparing the theoretical results obtained with the experimental ones. Moreover, the mesomorphic behavior and the nematic phase stabilities for each of the series have been predicted through the use of the calculated local charge distributions and dipole moments. (c) 2007 Elsevier B.V. All rights reserved.