RRKM stands for RiceRamspergerKasselMarcus. RRKM theory provides the means to evaluate the rate coefficient of a unimolecular reaction:
where:
 E = given internal energy for which the rate coefficient is estimated
 E_{0} = critical energy of the unimolecular reaction
 S^{*}(E  E_{0}) = number of states of the transition state in the energy range E  E_{0}
 r(E) = density of states of the equilibrium geometry at energy E
 h = Plank's constant
How to use the calculator:
 Input the critical energy of the unimolecular reaction (Critical Energy field) and the maximum energy (Maximum Energy field) for RRKM calculations (cm^{1});
 Input the vibrational frequencies of the molecule for the equilibrium geometry (Equilibrium Geometry field) and for the transition structure (Transition Structure field) (cm^{1}). The calculator expects one frequency per line.
 Click RRKM Curve button to calculate the RRKM rate coefficients for the internal energy interval [Critical Energy .. Maximum Energy]. Do not expect an output after clicking this button; to get any data, go to the next step.
 To find the rate coefficient corresponding to a given internal energy, input the energy value(s) (Energies field) and click Rate Coeffs button.
 To find the internal energy necessary for the reaction to proceed at a given rate, input the rate coefficient value(s) (Rate Coeffs field) and click Energies button.
The calculator is based on the BeyerSwinehart direct count method. This is a good place to thank again Professor John Christie from La Trobe University (Bundoora, Australia) for helping me understand this method.
We used this calculator to estimate the Internal Energy of Ions Generated by MALDI.
Reference: G. Luo, I. Marginean, A. Vertes, Internal Energy of Ions Generated by MatrixAssisted Laser Desorption Ionization, Anal. Chem. 2002, 74, 61856190.

You might need to install the Java Virtual Machine to see the above applet.
Reference: T. Baer, P.M. Mayer, J. Am. Soc. Mass Spectrom., 1997, 8, 103115. 