Monte Carlo Simulation Affects Convergence of Differential Evolution: A Case of Optical Response Modeling

It is known that the protein surrounding, as well as solvent molecules, has a significant influence on optical spectra of organic pigments by modulating the transition energies of their electronic states. These effects manifest themselves by a broadening of the spectral lines. Most semiclassical theories assume that the resulting lineshape of an electronic transition is a combination of homogeneous and inhomogeneous broadening contributions. In the case of the systems of interacting pigments such as photosynthetic pigment–protein complexes, the inhomogeneous broadening can be incorporated in addition to the homogeneous part by applying the Monte Carlo method (MCM), which implements the averaging over static disorder of the transition energies. In this study, taking the reaction center of photosystem II (PSIIRC) as an example of a quantum optical system, we showed that differential evolution (DE), a heuristic optimization algorithm, used to fit the experimentally measured data, produces results that are sensitive to the settings of MCM. Applying the exciton theory to simulate the PSIIRC linear optical response, the number of minimum required MCM realizations for the efficient performance of DE was estimated. Finally, the real linear spectroscopy data of PSIIRC were fitted using DE considering the necessary modifications to the implementation of the optical response modeling procedures.