The accurate modelling of absorption lineshapes of chromophores in the condensed phase is highly challenging, as both vibronic effects and explicit couplings between the pigment and its environment have to be accounted for. A variety of methods with different shortcomings and computational complexities have been developed in recent years, ranging from semi-classical ensemble approaches, Franck-Condon approaches, hybrid methods like the ensemble plus zero temperature Franck-Condon (E-ZTFC) approach, to fully dynamic descriptions of the lineshape in terms of quantum correlation functions of the energy gap. In this work, we apply these different approaches to a series of small model systems, such as a generalized form of the Brownian Oscillator Model (GBOM), the Morse oscillator and the ground- to S1 state transition of Phenolate. We find that the fully dynamic approach based on energy gap correlation functions delivers the best performance over a range of model systems for moderate solvent coupling strengths, with the added advantage that the approach allows for a straightforward inclusion of vibrational couplings between the chromophore and its environment. We furthermore demonstrate the strengths of the dynamic approach on a number of realistic systems, including the PYP chromophore and Methylene blue in aqueous solution. We find that non-linear terms in the energy gap fluctuations that go beyond the commonly used 2nd order cumulant approximation can contribute significantly to the simulated lineshapes. These non-linear effects are enhanced by couplings of chromophore vibrational modes to dynamic polarization effects of the solvent environment, requiring a full quantum mechanical treatment of large parts of the solvent environment to obtain accurate results.

Static and dynamic approaches to computing spectral lineshapes

2019 Fall National Meeting

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