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We report a fully quantum-dynamical study of the intramolecular vibrationalenergy redistribution (IVR) in the electronic ground state of carbonyl sulfide(OCS), which is a prototype of an isolated many-body quantum system with stronginternal couplings and non-Rice-Ramsperger-Kassel-Marcus (RRKM) behavior. Wepay particular attention to the role of many-body localization and the approachto thermalization, which currently are topics of considerable interest, as theypertain to the very foundations of statistical mechanics and thermodynamics. Weemploy local-mode (valence) coordinates and consider initial excitationslocalized in one local mode, with energies ranging from low to near thedissociation threshold, where the classical dynamics have been shown to bechaotic. We propagate the nuclear wavepacket on the potential energy surface bymeans of the numerically exact multiconfiguration time-dependent Hartree methodand employ mean local energies, time-dependent and time-averaged populations inquantum number space, energy distributions, entanglement entropies, localpopulation distributions, microcanonical averages, and dissociationprobabilities, as diagnostic tools. This allows us to identify a continuouslocalization $\rightarrow$ delocalization transition in the energy flow,associated with the onset of quantum chaos, as the excitation energy increasesup to near the dissociation threshold. Moreover, we find that at this energyand $\sim$1 ps the molecule nearly thermalizes. Furthermore, we observe thatIVR is so slow that the molecule begins to dissociate well before suchquasi-thermalization is complete, in accordance with earlierclassical-mechanical predictions of non-RRKM behavior.

Quantum dynamics of the intramolecular vibrational energy redistribution in OCS: From localization to quasi-thermalization