Surface Effects in Cluster Calculations of Energy Profiles of Muonium in Diamond

New and more accurate energy profiles for interstitial muonium in diamond clusters (${\mathrm{C}}_{10}$ ${\mathrm{H}}_{16}$ to ${\mathrm{C}}_{30}$ ${\mathrm{H}}_{40}$) have been determined by ab initio and approximate ab initio calculations, and have been critically compared to previous theoretical work. The energy profiles are shown to be extremely sensitive to the position of the surface atoms used to terminate the cluster. It is established that if the equilibrium carbon-hydrogen bond length is used to terminate the cluster the energy profiles converge rapidly with cluster size, but the use of any other value for $d_{\mathrm{C}-\mathrm{H}}$ leads to qualitatively incorrect energy profiles for small clusters.