The reactions taking place in the interstellar medium, in the radiation damage of biological tissues or inorganic materials, and in the imaging of large molecules with novel light sources have a complexity that eludes any ab-initio description. Yet, our grasp of these important processes depends on our ability to predict, in a quantitative way, how molecular aggregates break down into fragments when excited by fast projectiles, ionizing radiation, or intense laser pulses. Here, the Microcanonical Metropolis Monte Carlo method (M3C) is presented as a valid way to study the fragmentation of arbitrary aggregates in the gas phase, from isolated molecules to atomic and molecular clusters or complex biomolecules. The method follows a random walk sampling in the state vector (vibrational energy, relative orientation and positions, and chemical composition of the fragments) to estimate the density of states of the system and to calculate the observables. The validity and usefulness of the M3C method are demonstrated by simulating mass-spectrometry experiments on small molecules and by interpreting the experimental fragmentation patterns of highly excited small carbon clusters and hydrogenated carbon clusters.
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