3) The Rise-Ramsperger-Kassel-Marcus (RRKM) theory is a probabilistic model allowing to calculate k(E), i.e., how the dissociation rate k depends on the internal energy E (Figure ). RRKM calculations require knowing the threshold E0, but also the vibrational frequencies of the precursor ion (this is easy to calculate and can be measured in limited cases) and the vibrational frequencies of the transition state (this harder to calculate and cannot be measured). RRKM calculations are required to determine Eapp. 4) Internal energy distributions P(E): not all ions have the same internal energy E. An ion population has an internal energy distribution: a distribution of probabilities to have an energy E. <Eint> is the average energy of the internal energy distribution P(E). The most common type of internal energy distribution is the Boltzmann distribution, to which a temperature can be associated. Ideally, the internal energy distribution should be a symmetrical Boltzmann distribution but in practice, with different environments, pressure and sources, the shape of the distribution can deviate from the Boltzmann distribution.