The survival yield method (Figure ) is the simplest way to determine the internal energy distribution: the survival yield (SY = fraction of intact precursor ion) is measured for a set of thermometer ions with similar DOF but different Eapp. For each experimental condition, the SY of each molecule is plotted as a function of Eapp (Figure ). The first derivative gives P€ (Figure ). Many groups used E0 values instead of Eapp values, because Eapp would require RRKM calculations and knowing . With this simplification, the internal energies are systematically underestimated, but comparisons between different sources or conditions on the same instrument can still be made. Moreover, the SY curve is fitted by a sigmoid and this regression function is derived to giv€(E). However, fitting by a symmetric sigmoid will automatically give a symme€c P(E). Revealing the asymm€y in P(E) requires using asymmetric sigmoid functions and having enough data to do so ). In the last 20 years, everyone used symmetric sigmoids, but note that this assumption is not justified when internal energy distributions deviate from the Boltzmann distribution. 6) The characteristic temperature (Tchar) of an ion is the temperature of the Boltzmann distribution that would give the same survival yield as the experimental one . In contrast to the survival yield method, the characteristic temperature can be calculated for each thermometer ion. Characteristic temperatures obtained for a series of thermometer ions having the same number of DOF usually agree. Tchar can be obtained by programming the RRKM and P(E) functions. The MassKinetics software includes the Tchar determination, but also more advanced master equation modeling of internal energy distributions, adapted to MS/MS .