Two distinct areas within theoretical chemical physics are investigated in this dissertation. First, the dynamics of collinear exchange reactions is treated within a semiclassical Gaussian wavepacket (GWP) description. Second, a corrected effective medium (CEM) theory is derived which yields: (1) a one-active-body description of the binding energy between an atom and an inhomogeneous host; and (2) an N-active-body description of the interaction energy for an N atom system;To properly treat the dynamics of collinear exchange reactions, two extensions to the previous methodology of GWP dynamics are presented: (1) evaluation of the interaction picture wavefunction propagators directly via the GWP solution to the time-dependent Schrodinger equation; and (2) use of an expansion of GWPs to represent the initial translational plane wave. This extended GWP dynamical approach is applied to the H + H[subscript]2 collinear exchange reaction using the Porter-Karplus II potential energy surface;A one-active-body CEM theory is derived (denoted CEM-1) which describes binding between a single atom and an inhomogeneous host. The zeroth order term of the interaction energy is provided by a self-consistent calculation of the embedding energy of the atom into spin-unpolarized jellium. Higher order terms provide corrections of two sorts: (1) the Coulomb interaction between the charge densities on the atom and the host; and (2) the difference in kinetic-exchange-correlation energies between the atom/inhomogeneous host system and the atom/jellium system. The CEM-1 method is used to evaluate the interaction energies for: (1) H atom embedded into spin-polarized jellium; (2) some H atom containing diatomic molecules; and (3) H atom chemisorption on Ni(100), Cu(100), and Fe(110);The CEM-1 theory is then extended to provide an N-active-body theory (denoted CEM-N) where each atom in the N atom system is allowed to interact with the remaining (N - 1) atoms in a simultaneous fashion. The CEM-N method is tested by considering a variety of homonuclear diatomic molecules. These results illustrate the need for a new set of "covalent" embedding energies which are provided.