Towards the understanding of molecular properties in condensed phases from computer simulations and quantum mechanical calculations : methodological investigation and applications to water and related systems

• The present thesis summarizes some aspects in the studies of molecular properties by combining molecular dynamics computer simulations and quantum mechanical calculations. Applications to water and related system are overviewed. First, method of combined quantum mechanical and molecular mechanical (QM/MM) calculations is outlined. Molecular dynamics simulations on water systems using combined QM/MM potentials are reviewed. In the simulations, the QM calculations are carried out at the ab initio Hartree-Fock level and TIP3P water models are used to represent the MM water molecules. The main focus of the studies is on the combined QM/MM coupling potentials. The simulation results show that the QM/MM coupling is very sensitive to the Lennard-Jones parameters used on the QM atoms. When the TIP3P Lennard-Jones parameters are used for the QM atoms, the QM/MM coupling is found to be too strong. However, when the LennardJones parameters on the QM atoms are optimized according to the corresponding QM/MM calculations of water dimer, the QM/MM coupling seems to be too weak. Therefore, we believe that the Lennard-Jones parameters on the QM atoms optimized from the corresponding QM/MM calculations for some molecular complexes cannot reliably be used in the simulation of the condensed phases. A modified QM/MM coupling for water systems is found by scaling down the MM charges appearing in the coupling potential. Studies show that the new QM/MM coupling potentials can be used satisfactorily in the combined Hartree-Fock QM/MM molecular simulations of water from ambient to supercritical conditions.In the thesis, a self-consistent charge approach developed for the study of the electronic properties of a single molecule in condensed phases is also outlined. In the approach, only the Coulomb interaction between the molecules is considered. Therefore, it is suitable for the study of molecular properties in systems where Coulomb interaction is predominant, such as liquid water, methanol, etc. As an application, the electronic properties of water molecules in water clusters and liquid water are studied. The average water dipole moment in liquid water is found to be 2.65 D. Significant changes in quadrupole moment and polarizability are also found along the water plane in the direction perpendicular to the axis bisecting the H-O-H bond angle. A further theoretical derivation based on the interaction model is presented to find the atomic charge model suitable for the MM force fields. The derived charge model shows that the appropriate atomic charge corresponds to the average value of the electrostatic potential derived charge for an isolated molecule and that obtained for the molecule in the interaction system. Applications to liquid water and methanol show that the obtained charges are similar to those found in the widely used MM force field models, such as SPC, TIP3P, OPLS, etc.   

Ämnesord

NATURVETENSKAP  -- Kemi -- Fysikalisk kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Physical Chemistry (hsv//eng)

Nyckelord

Physical Chemistry

fysikalisk kemi

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