Calculating molecular properties in realistic environments

Abstract

This thesis focuses on how absorption properties of molecules are influenced by their environment and how this can be calculated accurately. Calculations have been performed with a polarizable embedding (PE) multiscale model. The environment is described classically by charges and electric multipoles for the permanent electrostatics and polarizabilities for polarization interactions. Density-functional theory (DFT) and approximate singles and doubles coupled-cluster theory (CC2) are used to describe the electronic structure of the molecules. The results indicate that the effects of environmental polarization on electronic and vibrational properties are significant and that the employed PE model is accurate in cases where electrostatic interactions dominate. A large part of the environment needs to be described explicitly for converged molecular properties, especially since polarization interactions range over a long distance. However, accurate embedding parameters for the electrostatic and polarization interactions are important mainly for the closest environment of a chromophore. This enables a reduction of the computational cost of obtaining embedding potentials without sacrificing accuracy. For localized properties, PE is to be preferred over a cluster approach because the latter is severely limited by the possible size of the molecular system. For calculation of two-photon absorption (TPA), DFT and CC2 give qualitatively but not quantitatively similar results. Finally, it is shown that the comparison between calculated TPA cross sections and other experimental or theoretical work is challenging. The presented works contribute to the realistic description of a molecular environment with the accurate prediction of molecular properties in chemical environments as ultimate goal.