Reduced vibronic coupling density (RVCD) and reduced atomic vibronic coupling density (RAVCD) are introduced in order to discuss the origin of vibronic (electron-vibration) couplings in a molecule. As an example, the RVCDs and RAVCDs of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) cation are presented. The strongest vibronic coupling of the C=C stretching mode originates from large electron-density difference on the C=C bond. (C) 2010 Elsevier B.V. All rights reserved.