Methodologies are presented in which population dynamics are evolved in the exciton basis and spatiotemporal movement of excitations is subsequently obtained by projection to the site basis. Fluctuations of system eigenstates are explicitly included through vibrations of the chromophores, which are parametrized by ab initio calculations. Two limiting cases of dynamics are considered, namely, the incoherent regime, where state populations correspond to ensembles of classical Landau–Zener (LZ) trajectories, and the coherent regime, where the density matrix is propagated by the quantum Liouville equation (QLE). For QLE simulations, population dynamics show that bacteriochlorophyll a1 and a2 effectively act as a single unit at 77 K but as independent chromophores at 300 K. Population beatings for the lower energy exciton states are considerably slower at physiological temperatures, thus assisting transfer to the sink. Results from LZ trajectories indicate that, within the classical picture, higher temperatures result in a lower probability of the exciton reaching the sink. A broadening of the excitonic spectrum at high temperature alters the pathways of the excitons in the LZ formalism and also increases the possibility of trapping. This study supports the view that a coherent mechanism may assist EET at physiological temperatures since the trapping of excitations in intermediate energy sites is prevented. Furthermore, delocalized vibrations (i.e., superpositions of independent oscillators) are found to assist energy transfer at short times.

Experimental estimates of photolytic efficiency (yield per photon) for photodissociation and photodesorption from water ice range from about 10–3 to 10–1. However, in the case of photodissociation of water in the gas phase, it is close to unity. Exciton dynamics carried out by a quantum mechanical time-dependent propagator shows that in the eight most stable water hexamers, the excitation diffuses away from the initially excited molecule within a few femtoseconds. On the basis of these quantum dynamics simulations, it is hypothesized that the ultrafast exciton energy transfer process, which in general gives rise to a delocalized exciton within these clusters, may contribute to the low efficiency of photolytic processes in water ice. It is proposed that exciton diffusion inherently competes with the nuclear dynamics that drives the photodissociation process in the repulsive S1 state on the sub-10 fs time scale.Keywords: astrochemistry; atmospheric chemistry; quantum dynamics; resonance energy transfer; water clusters; water hexamers; water splitting;