Modelling of polarization by molecular force fields: Further development of the NEMO potential

Sammanfattning: Understanding of intermolecular interactions are important in order to gain insight into a large number of physical processes. This thesis concerns itself with theoretical modelling of such interactions. In particular there is a focus one of interaction contributions called polarization. This interaction is sometimes ignored, but is a real and often important effect. For a model to be able to describe situations with varying chemical environments (for instance at surfaces) it must be included. The research presented in this thesis can be divided into three different categories; modelling of intramolecular polarization, modeling including higher-order polarizabilities and description of the chemical bond using a charge capacitance. Of these three categories the first two are concerned with improving the description of intermolecular interactions whereas the last part is concerned with the chemical bond. For intramolecular polarization a model based on previous work (J. Phys. Chem., 100:6950, 1996) is extended to include a damping. It is shown that this model improves the description of the molecular dipole moment when rotating dihedral angles, and that the model gives better interaction energies. In the study of higher-order polarizabilites a method for inclusion of localized dipole-quadrupole polarizabilities are presented, and it is shown that inclusion of this polarizability gives a better description of the induced potential around a molecule. In a second study localized dipole-quadrupole and quadrupole - quadrupole polarizabilities are included, and it is shown that inclusion of these gives better potential energy curves for the formaldehyde dimer. It is also shown that using local multipoles based on MP2 densities gives a correct behavior at larger separations. In the last research papers the charge capacitance presented and studied. The charge capacitance is based on the charge part of the localized dipole-dipole polarizability in the LoProp method. It is demonstrated that the charge capacitance is corresponds well with classical bonding situations. Furthermore it is shown that the charge capacitance is correlated with the strength of the agostic interaction in a series of molecules. It is shown that the charge capacitance correlates with other descriptors of bond strength in molecules containing carbon - carbon, carbon - silicon and silicon - silicon bonds. It is also demonstrated that the charge capacitance can be used, together with other descriptors, to predict the regio-selectivity of aromatic nucleophilic substitution