Modification with thiomolybdate results in the attachment of spatially separated MoS42- groups to the surface of CdS. This produces the red shift of excitonic absorption and emission bands and splitting of the trapped emission band. Because of the spatial gap between the thiomolybdate molecules of the modifying layer, the interaction of individual groups of the modifier with the CdS core must be considered. This was accomplished by using the density functional theory (DFT) description of a MoS42--CdS model cluster. Geometry, dipole moment, and localization of calculated molecular orbitals allow for the consistent description of optical effects observed for thiomolybdate-modified CdS nanoparticles, such as the red shift of adsorption and emission bands, enhancement of excitonic emission, and splitting of the trapped emission band. A good agreement between DFT calculated and experimental FTIR spectra has been obtained.