David Ceperley University of Illinois at Urbana-Champaign Quantum Monte Carlo applied to electronic ground state problems is able to perform accurate "benchmark" quality results. However, applications outside of perfect bulk solids have been quite limited. In this talk I will discuss some of the limitations to applying QMC to defects, in particular, the difficulties in reaching large enough system size, the computer requirements to reach sufficient accuracy, the difficulties with pseudo potentials, problems in estimating the geometry and the needed accuracy of the trial wave function. To minimize the computation effort, we have recently developed and tested a method to calculate the electronic energy density[1]. This can be used to focus the computational effort on a specific spatial region, thus improving the efficiency of calculating defect energies. Using a model of delta doped silicon (where dopant atoms form a thin plane) we showed how interfacial energies can be calculated more efficiently . The basic physical properties of germanium have gained renewed interest as it has become a foundational material for various technologies, such as fiber optics, infrared optics, solar cells, nuclear radiation detectors, and CMOS devices. Recent efforts to fabricate n-channel Ge MOSFETS have been challenged by enhanced vacancy mediated dopant diffusion. Stimulating interstitial generation via irradiation or other means promises improved control of this undesired migration. The fundamental properties of germanium self-interstitials have been the subject of relatively few theoretical studies, inviting the application of accurate QMC methods that have had prior success with the silicon self-interstitial. We are estimating with QMC the relative stability of the neutral self-interstitial in several configurations and show how the efficiency of such calculations can be improved using the energy density[2].