Quantum decoherence: A consistent histories treatment of condensed-phase non-adiabatic quantum molecular dynamics

We address the issue of quantum decoherence in mixed quantum classical simulations. We demonstrate that restricting the bath paths to a single stationary path which connects an initial quantum state to a final quantum state affects a coarse graining of the quantum subspace which leads to a macroscopic loss of quantum coherence. The coarse graining can be described in terms of reduction mappings of the density matrix of the reduced quantum system + stationary bath path. Application of the present model to various prototypical condensed-phase chemical problems reveals that non-adiabaticity is extremely sensitive to the decoherence timescale. Furthermore, we derive how to obtain quantum coherence timescales from realistic mixed quantum classical simulations and use this information to compute the non-radiative lifetimes for an excess electron in H2O and D2O. We demonstrate that subtle differences in the quantum coherence times provide a rationalization for a long-standing puzzle regarding the lack of experimentally observed isotopic dependence of the non-radiative lifetime of a photoexcited electron in H2O and D2O.