Evaluation of the intermolecular interactions in biological systems using a theoretical pseudoatom databank
Accurate and fast evaluation of interactions in molecular systems is still one of the most challenging tasks in the field of macromolecular chemistry and biology. To calculate the interaction energies for macromolecules, the transferability is of great importance. The construction of a theoretical databank of transferable pseudoatoms is described in this thesis. The databank gives very consistent results in electron densities in different structures. The excellent transferability of the theoretical pseudoatoms is demonstrated by the agreement between primary and databank densities. The exact potential/multipole model (EP/MM) method treats the electrostatic interactions between a pairs of atoms with numerical evaluations of the exact Coulomb integral within a certain critical distance, while the Buckingham-type multipole approximation is performed for large separation. Comparing with point charge model, such as MMFF94 force field results, and full integration over molecular wavefunctions, the EP/MM method combined with the theoretical databank of transferable pseudoatoms offers better accuracy than the former, at significantly less computational cost than the latter. The total intermolecular interactions are calculated by the sum of electrostatic term which is based on the databank parameters, and the exchange-repulsion and dispersion terms which are based on the atom-atom potential parameters from the fitting of Symmetry Adapted Perturbation Theory (SAPT) calculated energies, giving very good results by comparing theoretical calculations with different levels of theory and basis sets. This new approach can be exploited to great potential for the evaluation of the interaction energies for macromolecules when experimental data are not available and the computational cost is too expensive.