Theoretical investigations of open-shell systems: 1. Spectral simulation of the 2s(2)p(2) (2)D <- 2s(2)2p (2)P(o) valence transition in the boron diargon cluster, and 2. Quantum Monte Carlo calculations of boron in solid molecular hydrogen

This dissertation is concerned in part with the construction of accurate pairwise potentials, based on reliable ab initio potential energy surfaces (PES's), which are fully anisotropic in the sense that multiple PES's are accessible to systems with orientational electronic properties. We have carried out several investigations of B (2s 22p 2Po) with spherical ligands: (1) an investigation of the electronic spectrum of the BAr₂ complex and (2) two related studies of the equilibrium properties and spectral simulation of B embedded in solid pH ₂. Our investigations suggest that it cannot be assumed that nuclear motion in an open-shell system occurs on a single PES.; The 2s2p2 2 D ← 2s22p 2Po valence transition in the BAr ₂ cluster is investigated. The electronic transition within BAr ₂ is modeled theoretically; the excited potential energy surfaces of the five-fold degenerate B(2s2p2 2D) state within the ternary complex are computed using a pairwise-additive model.; A collaborative path integral molecular dynamics investigation of the equilibrium properties of boron trapped in solid para-hydrogen (pH₂) and a path integral Monte Carlo spectral simulation. Using fully anisotropic pair potentials, coupling of the electronic and nuclear degrees of freedom is observed, and is found to be an essential feature in understanding the behavior and determining the energy of the impure solid, especially in highly anisotropic matrices.; We employ the variational Monte Carlo method to further study the behavior of ground state B embedded in solid pH₂. When a boron atom exists in a substitutional site in a lattice, the anisotropic distortion of the local lattice plays a minimal role in the energetics. However, when a nearest neighbor vacancy is present along with the boron impurity, two phenomena are found to influence the behavior of the impure quantum solid: (1) orientation of the 2p orbital to minimize the energy of the impurity and (2) distortion of the local lattice structure to promote an energetically favorable nuclear configuration.; This research was supported by the Joint Program for Atomic, Molecular and Optical Science sponsored by the University of Maryland at College Park and the National Insititute of Standards and Technology, and by the U.S. Air Force Office of Scientific Research. (Abstract shortened by UMI.)...