Decoherent Histories and Non-adiabatic Quantum Molecular Dynamics

The role of quantum coherence loss in mixed quantum-classical dynamical systems is explored in the context of the theory of quantum decoherence introduced recently by Bittner and Rossky. (J. Chem. Phys. {\bf 103}, 8130 (1995)). This theory, which is based upon the consistent histories interpretation of quantum mechanics, introduces decoherence in the quantum subsystem by carefully considering the relevant time and length scales over which one must consider the effects of phase interference between alternative histories of the classical subsystem. Such alternative histories are an integral part of any quantum-classical computational scheme which employ transitions between discrete quantum states; consequently, the coherences between alternative histories have a profound effect on the transition probability between quantum states. In this paper, we review the Bittner-Rossky theory and detail a computational algorithm suitable for large-scale quantum molecular dynamics simulations which implements this theory. Application of the algorithm towards the relaxation of a photoexcited aqueous electron compare well to previous estimates of the excited state survival time as well as to the experimental measurements.Comment: 22 pages, 3 figure

Calculations of the Exciton Coupling Elements Between the DNA Bases Using the Transition Density Cube Method

Excited states of the of the double-stranded DNA model (A)$_{12}\cdot$(T)_{12} were calculated in the framework of the exciton theory. The off-diagonal elements of the exciton matrix were calculated using the transition densities and ideal dipole approximation associated with the lowest energy $\pi\pi^{*}$ excitations of the individual nucleobases obtained from TDDFT calculations. The values of the coupling calculated with the transition density cubes (TDC) and ideal-dipole approximation (IDA) methods were found significantly different for the small inter-chromophore distances. It was shown that the IDA overestimates the coupling significantly. The effects of the structural fluctuations were incorporated by averaging the properties of the excited states over a large number of conformations obtained from the MD simulations