The work in this thesis concerns the application, evaluation, and development of electronic structure methods. The applications projects concern silicon carbide clusters, and the reaction of acetylene with the silicon surface. SiC3 and Si2C2 are studied with multi-reference methods, and the global minimum geometries are identified for both. The reaction of acetylene on the Si(100)-(2 x 1) surface is studied using the surface integrated molecular orbital/molecular mechanics method, a combined quantum mechanics/molecular mechanics embedded cluster method. Multi-reference second order perturbation theory is used in both of these studies to include dynamic correlation in determining relative energies and barrier heights. In the following chapter the size consistency of multi-reference perturbation theory is evaluated; specifically the MRMP2 and MCQDPT2 implementations of Hirao and Nakano. The final topic concerns a contribution to generalizing the effective fragment potential (EFP) solvent/liquid model to allow the straightforward generation of an EFP for any solvent or molecule. Approximate energy and energy derivative expressions are reported. Implementation of these derivative expressions will allow optimization of mixed ab initio and EFP clusters, including aqueous and non-aqueous solutions.