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    Theoretical study of weakly interacting systems : noble gas compounds : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Physics at Massey University, Albany, New Zealand
    (Massey University, 2023) Florez Hincapie, Edison Ferney
    The dispersion bonds − also known as London or Van der Waals interactions − are usually considered the weakest and the least important of the several types of chemical bonding models known. However, dispersion interactions play a crucial role in chemistry; particularly, in defining functions and structural stability of proteins and Watson-Crick pairs in biochemical systems, to name just a few. Through the Periodic Table, noble gas aggregates are one of the prime examples of systems strongly influenced by dispersion interactions. In addition, from helium − the smallest and most unreactive − to radon, periodic trends emerge; the atomic mass/radius and the dispersion interactions increase, resulting in increasing melting points, boiling points, enthalpies of vaporization, etc. However, Oganesson (Og, Z=118) − the heaviest noble gas and element synthesized at the limit of nuclear mass and charge − may behave differently from what would be predicted by simple extrapolations in the Periodic Table. This is due to relativistic effects. Those effects are more pronounced in heavier elements (high nuclear charge Z) and significantly influence both chemical and physical properties. The research presented here is divided into three parts. First, we explore the behavior of neon clusters under high magnetic fields in the range of 0 to 7.5x10⁵ Tesla. Under these extreme conditions, atoms and molecules reveal exotic chemical characteristics such as squeezed and twisted structures as dispersion interactions are affected by the so-called perpendicular paramagnetic bonding giving rise to molecules and materials that do not exist on Earth (but in environments of white dwarfs and magnetic stars). Our results show that, regarding the field-free case, there is an energetic stabilization for the neon interaction in a magnetic field, leading to enhanced melting temperatures of more than 70% and reducing the entropy of the system, squeezing the structures perpendicular to the applied magnetic field. Second, we analyzed the chemical nature of Flerovium clusters and their noble gas-like behavior upon melting. Here, we studied closed-shell flerovium in detail to predict solid-state properties including the melting point from a decomposition of the total energy into many-body forces derived from relativistic coupled-cluster and density functional theory. Our results show that the noble gas behavior of flerovium enhances resistance to bond formation. Flerovium atoms are only weakly bound, less compared to mercury, but more than in xenon. This makes the accurate prediction of phase transitions very difficult. Nevertheless, we made the first prediction by Monte-Carlo simulation estimates the melting point at 284 K (std. 50 K). Finally, we studied the structure, stability, and chemical bonding of fluorides noble gas compounds, NgFₙ (n=2, 4, 6), where Ng comes from Argon, Krypton, Xenon, Radon, and Oganesson. The heaviest element, Oganesson, unlike all other noble gas compounds, is enhancing the tendency to adopt a tetrahedral local environment. These results indicate that there may be a partial role reversal of the elements Fl and Og important in the future of atom-at-a-time chemistry. Oganesson di- and tetra-fluorides are stable with and without relativistic effects, whereas hexafluoride is unstable. This creates such a radical departure from periodic group trends that the rules of the periodic table appear to be broken by relativity, suggesting an end to periodicity.
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    Melting temperatures of the noble gases from ab-initio Monte Carlo simulations : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Physics at Massey University, Albany, New Zealand
    (Massey University, 2019) Smits, Odile R.
    This thesis describes simulations to determine the melting temperatures of the noble gases based on first-principles ab-initio methods. The melting temperatures of bulk krypton, xenon, radon and oganesson are determined using parallel-tempering Monte Carlo with the interaction potential approximated by two- and three-body contributions. The employed interaction potentials are obtained from relativistic coupled cluster theory including spin-orbit coupling and are the most accurate ab-initio potentials to this date. These potentials are fitted to computationally efficient functions utilized to calculate the interaction energy during the Monte Carlo melting simulation. Two different techniques of obtaining the melting temperature are presented. First, the melting temperature is studied by simulating finite clusters in a canonical ensemble. The melting temperature is then deducted from extrapolation of the finite cluster results to the bulk. Second, the melting temperature is determined by direct sampling of the bulk using cells with periodic boundary conditions in the isobaric-isothermal ensemble. Upon correction for superheating, an excellent agreement to the melting temperatures obtained from cluster simulations is obtained. The numerically determined melting temperatures of krypton and xenon are in close agreement with available experimental data. That is, for krypton a melting temperature of 109.5 K and 111.7 K is obtained for cluster and periodic simulations respectively, which is approximately 5 Kelvin lower than the corresponding experimental value of 115.78 K. The melting point of xenon is determined to be 156.1 K and 161.6 K respectively, which compares to the experimental value of 161.40 K. The long debated value of the radon melting temperature of 202 K is confirmed by our simulations (200 K for both techniques). And finally, the melting point of oganesson is determined to be 330 K and therefore surprisingly high compared to the other rare gases. This implies that oganesson is a solid at room temperature. Furthermore, an analytical formula to compute the temperature of the solid-liquid phase transition based on the analytically expressed bulk modulus and interaction potential is presented, and the superheating correction factor is evaluated.