Yes it is very common particularly in the more rigid type of molecule. The best example is chlorophyll and this overlap of absorption and emission leads to energy transfer in photosynthesis. The reason for the overlap is that the excited state potential energy profile is very similar in shape but slightly shifted (as displacement) to that of the ground ...
Allright, guys. All I wanted happens to be stochastic simulation.
Here is the list of sample algorithms:
Gillespie SSA - original.
Tau-leaping - evaluate time step and then out of that reactions and substances amount.
R-leaping - vise versa. Good for big systems with only a few relatively highly reactive species.
S-leaping - hydrid of R-leaping and Tau-...