| The nuclear-electronic orbital (NEO) method was modified and extended to positron systems for studying mixed positronic-electronic wavefunctions. These methods include: Hartree-Fock (HF); second-order Moller-Plesset perturbation theory (MP2); configuration interaction (CI); complete active space self-consistent field (CASSCF); and full configuration interaction (FCI). The methodology for calculating positron-electron annihilation rates based on NEO-HF and NEO-MP2 wavefunctions was also implemented. Positronic and electronic basis sets were optimized at the NEO-FCI level for the positronium hydride (PsH) system and used to compute NEO-MP2 energies and annihilation rates. The effects of basis set size on correlation energies captured with the NEO-MP2 and NEO-FCI methods are compared and discussed.; Equilibrium geometries and vibrational energy levels were computed for the LiX and e+LiX (X = H, F, Cl) systems at the MP2 and NEO-MP2 levels. Anharmonic effects were included by fitting the computed potential energy curves (PECs) to a Morse potential function. It was found that anharmonicity plays a significant role, specifically in the differences between the vibrational energy levels of the LiX and e+LiX systems. The implications of these results with respect to vibrational Feshbach resonances (VFRs) for these systems is discussed.; The positron lifetime in pressed samples of K2B12H 12·CH3OH was measured to be 0.2645+/-0.0077 ns. This result is interpreted with quantum mechanical calculations of B 12H122- and e+B12H 122-. Calculations reveal a spherically symmetric positronic wavefunction, with a peak in the positron density at the outside edge of the hydrogen atom cage. The experimentally determined annihilation rate corresponds to an effective number of electrons of 1.88, or 0.94 of the two electrons present in the B12H122- dianion, indicating that there is significant positron density both inside and outside of the B12H122- cage. |
