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Path integrals have, over the years, proven to be an extremely versatile tool for simulating the dynamics of open quantum systems. The initial limitations of applicability of these methods in terms of the size of the system has steadily been overcome through various developments, making numerical explorations of large systems a more-or-less regular feature. However, these simulations necessitate a detailed description of the system-environment interaction through accurate spectral densities, which are often difficult to obtain. Additionally, for several processes, such as spontaneous emission, one only has access to a rough estimation of an empirical timescale, and it is not possible to really define a proper spectral density at all. In this communication, an approach of incorporating such processes within an exact path integral description of other dissipative modes is developed through the Nakajima-Zwanzig master equations. This method will allow for a numerically exact non-perturbative inclusion of the degrees of freedom that are properly described by a bath using path integrals, while incorporating the empirical time scale through the Lindblad master equation. The cost of this approach is dominated by the cost of the path integral method used, and the impact of the Lindbladian terms is effectively obtained for free. This path integral Lindblad dynamics method is demonstrated with the example of electronic excitation transfer in a 4-site model of the Fenna-Matthews-Olson complex with the exciton has a propensity of being "lost" to the charge transfer state at the third chromophore. The impact of different time-scales of abstraction of the exciton is illustrated at no extra cost.
Amartya Bose (Tue,) studied this question.