An evaluation of model potential and pseudopotential methods for the calculation of interatomic potentials

This thesis presents an evaluation of the model potential and pseudopotential methods for the calculation of atom-atom interaction curves. At University College London, Peach has computed interatomic potential curves for several alkali-rare gas pairs using both the model potential and pseudopotential methods in a completely self-consistent manner. The value of these calculations is that any differences between the interatomic potential curves obtained from the model potential and pseudopotential methods can be attributed unambiguously to the theoretical representations of the Pauli exclusion principle which differentiate the two methods. The work of Peach thus allows the utility of these two representations to be compared directly and their effect on the quality of the resulting interatomic potentials to be assessed. The present study has developed a suite of computer programs designed to calculate observable quantities which are sensitive to the interatomic potentials. Calculations of the profiles, widths and shifts of the alkali resonance line when perturbed by rare gases, together with cross sections for fine structure state changing transitions within the alkali excited state multiplet, are reported and comparisons are made with experimental measurements of these observables. It is demonstrated that the theoretical representation of the Pauli exclusion principle has a significant effect on the quality of the resulting interatomic potentials. Observables calculated on the basis of interatomic potentials obtained via the model potential method are in better agreement with the available experimental data than those calculated on the basis of pseudopotential interaction curves. From this it may be inferred that the interatomic potentials obtained by the model potential method represent the actual interaction between alkali and rare gas atoms more accurately than do interatomic potentials obtained by the pseudopotential method. The results of this study provide strong evidence for the superiority of the model potential method for the calculation of interatomic potentials