To determine the energetic feasibility of the mechanisms involved in the generation of the fluorescent species in red fluorescent proteins LSSmKate1 and LSSmKate2 developed by Piatkevich et al. (Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 5369-5374 and J. Am. Chem. Soc. 2010, 132, 10762-10770), a potential energy scan for the respective reaction coordinates was performed in large cluster models including the surroundings of the chromophores, based on the respective crystallographic structures, using DFT and TDDFT. The predicted absorption wavelengths agree to within 5 nm with experiment, thus confirming the accuracy of the calculational level and modeling done. In both proteins, it was found that the adiabatic electronic state with the largest oscillator strength at the Franck-Condon region was not the one from which fluorescence could occur in the products. A diabatization procedure was used to determine an approximate photoactive state, based on selecting the state with the largest oscillator strength throughout. For LSSmKate1, this led to a rather flat potential energy profile but still did not predict a minimum in the product side. It is suggested that relaxation processes, absent from the model, could bring about such a minimum. LSSmKate2, on the other hand, clearly displays a favorable exoergic process in the photoactive state, and its double-proton transfer can be described as concerted but highly asynchronous, involving a barrier in the transfer of the first proton. In this way, the model provides strong support for the mechanism proposed for LSSmKate2. © 2012 American Chemical Society.