Complex sentence: For strong tunnelling to be possible, the odorant molecule must first fit in the receptor's binding site, and then it must have a vibrational energy mode compatible with the difference in energies between the two energy levels on the receptor.
Split into simpler sentences: For strong tunnelling to be possible, the odorant molecule must first fit in the receptor's binding site. Then it must have a vibrational energy mode compatible with the difference in energies between the two energy levels on the receptor.