First-principles calculations of the phonon modes of 4'-dimethylamino-N-methyl-4-stilbazolium tosylate in the terahertz region were performed using periodic density functional calculations. The calculated far-infrared (FIR) spectrum estimated from the phonon modes of the crystal did not correspond to the sum of the calculated spectra of the constituent cation and anion. The intramolecular modes of the methyl and sulfone group torsions and of pi-bridge bends were confirmed, including in-plane and out-of-plane intramolecular bending, with the intermolecular modes contributing to the FIR absorption of the collective peaks. The assignment of FIR-active modes provides fruitful ideas for the design and synthesis of promising derivatives. (c) 2006 Elsevier B.V. All rights reserved.