Orbital-vibronic coupling constants were calculated and analyzed using cluster models with D-6h symmetry at the B3LYP level of density functional theory to aid the understanding of the electron-phonon interaction mechanism in magnesium diborate, a new binary superconductor with T-c of 39 K. How the frontier orbitals couple with the E-2g modes of molecular vibration in the cluster models is discussed with an emphasis on the symmetry of the relevant molecular orbitals and vibrations.