Understanding and predicting one- and two-photon absorption properties of molecular complexes

Abstract

This thesis presents the development of theoretical models for the calculations of one- and two-photon absorption, and computational studies on solvated systems and biomolecules. The photon-absorbing chromophore is described by density functional theory, while the effects of the surroundings are taken into account by means of polarizable embedding models. The theory and implementation of a three-layered fully polarizable method is presented in this thesis. In this method, the short-range electrostatic potential due to the solvent is treated by a polarizable molecular mechanics force field, while the long-range effects are described by a dielectric continuum. This QM/MM/PCM implementation was tested on three organic molecules solvated in water and shown to converge faster with respect to system size compared to calculations using quantum mechanics/molecular mechanics (QM/MM) only. Further, the parallelization of the QM/MM module in the Dalton program is decribed, making it possible to do calculations on large molecular systems with the use of modern supercomputers. This implementation was used to calculate the one- and two-photon absorption properties in fluorescent proteins, demonstrating the importance of describing the protein surrounding the chromophore by a polarizable embedding.