Stochastinių metodų taikymas modeliuojant molekulinių sistemų fluorescencijos spektrus / ; Application of stochastic methods for modeling of fluorescence spectra of molecular systems.

Fluorescence is a powerful technique for studying molecular systems. It is central to resonant energy transfer (FRET), which is very efficient due to the non-radiative nature of the energy transfer. Modelling of open quantum systems is challenging, mainly because of the infinite number of thermostat degrees of freedom. Many calculations rely on perturbative methods or quantum-classical methods, so the development of accurate computational models of fluorescence spectra is still needed. The aim of this work was to apply stochastic computation method for fluorescence spectra modelling, to compare the results to those obtained by the method of hierarchical equations of motion (HEOM), and to verify the accuracy of the fluorescence spectra obtained by complex time Redfield theory. A stochastic method for modelling fluorescence spectra has been applied which is accurate for any spectral density of a thermostat. Its results are compared with the equally accurate results of hierarchical equations of motion. The fluorescence spectra obtained by the stochastic and hierarchical equations methods were in good agreement, which is to be expected since both methods are accurate. The accuracy of the complex time Redfield theory (ctR) was also tested. The stochastic calculations also showed that the ctR method, although accurate in calculating the absorption spectra, reproduce the fluorescence spectra accurately only when resonance coupling between the exited states of molecules J is low.