H/D isotope effects on the geometry and infrared spectrum of the protonated ammonia dimer

► Six-dimensional vibrational quantum simulation of strong hydrogen bond where the zero point energy is close to the rather high reaction barrier. ► Prediction of symmetrization due to zero point energy for normal and fully deuterated species. ► Prediction of secondary Ubelohde effect. ► Prediction of IR signature of deuteron stretching vibration as low as 250 cm−1.