MesoHOPS: simulations of multiple excitations interacting with molecular vibrations in large systems

What is it about?

Molecular vibrations influence the dynamics of electronic excitations on the 100s-of-nanometer scale, but accounting for the role of these vibrations makes simulations computationally demanding. By taking advantage of the fact that interacting with vibrations localizes electronic excitations onto a small handful of molecules, the adaptive Hierarchy of Pure States (adHOPS) method flexibly ignores unimportant terms in the equation-of-motion at each point in time, reducing the difficulty of calculations. In fact, adHOPS allows for size-invariant calculations: past a certain threshold, it is no more difficult to simulate a system that is even larger. In this paper, we extend adHOPS to systems where multiple excitations exist at the same time and add new features tailored to systems where vibrations reach equilibrium on ultrafast timescales. We present the MesoHOPS package as an open-source Python implementation of the method and investigate the effect of the simultaneous mobility of negatively-charged electrons and positively-charged holes on charge separation efficiency in a 1-dimensional model of an organic photovoltaic cell.

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Why is it important?

Despite the presence of many sophisticated computational methods, the transport of energy and charge in molecular materials on the 100s-of-nanometers scale remains poorly-understood. This makes it difficult to design devices like organic photovoltaic cells to be as efficient as possible. By introducing a method tailored to large length scales, we can better characterize the properties of these systems and discover new design principles.