Determination of intermolecular potential parameters in virial coefficients using ab-initio approach based on Gaussian type orbitals and applications

The determination of intermolecular interaction potentials has an extremely important place in the study of the physical and chemical properties of atoms and molecules. However, it is very difficult to determine the parameters involved in these interaction potentials both experimentally and theoretically. In this study, a new method for the determination of the parameters involved in intermolecular interaction potentials has been developed using advanced quantum mechanical approaches. In physics, many equations of state have been proposed to determine the thermophysical and thermochemical properties of an atomic and molecular system. One of these equations of state is the virial equation of state. By using the virial coefficients in this equation, the physical and chemical properties of molecular systems can be determined. While calculating the virial coefficients, intermolecular interaction potentials are used according to the structural properties of various molecules or atoms. In order to calculate the virial coefficients, the parameters of the intermolecular interaction potentials must be known. Therefore, precise calculation of the parameters involved in the intermolecular interaction potential is very important for the calculation of virial coefficients. Therefore, in this thesis, for the first time, a new approach for the determination of the intermolecular potential parameters from the intermolecular interaction energy data obtained using the Density Functional Theory (DFT) method, a quantum mechanical method, has been developed and presented. The method proposed in this study allows the determination of different intermolecular interaction potential parameters of atoms and molecules. In general, this approach has great advantages for the determination of all kinds of potential parameters of atoms and molecules. As an application, the Lennard-Jones (12-6) and Morse potential parameters of the molecule were first determined using the 6-31G basis set in the DFT method B3LYP function. In order to demonstrate the accuracy and precision of the determined potential parameters, some thermodynamic properties were applied. As an application, the speed of sound and heat capacities of the molecule in some temperature and pressure ranges were calculated. The results obtained were found to be in good agreement with the data in the literature. Keywords: Density Functional Theory; Lennard-Jones (12-6) potential; intermolecular potential parameters; Morse potential; speed of sound; virial equation of state