High‐level ab initio calculations were performed to determine accurate analytic potential energy functions for interactions a gas‐phase H‐atom has with H‐atoms attached to the diamond (111) surface and with C‐atom radical sites on this surface. The nonbonded potential between the gas‐phase H‐atom and H‐atoms attached to the surface was determined from coupled‐cluster ab initio calculations, including single, double, and perturbatively applied triple excitations [CCSD(T)], with the 6‐311++G(2df,p) basis set. The resulting nonbonded potential is nearly identical to that found previously from both theory and experiment for interactions between H‐atoms on different hydrocarbon molecules. In the ab initio calculations, a C‐atom radical site on the diamond surface is represented by a constrained tert‐butyl radical. Radial and small‐displacement angular potentials for a H‐atom interacting with this radical were determined from unrestricted quadratic configuration interaction calculations, with single, double and perturbatively applied triple excitations [UQCISD(T)], with the 6‐31G∗∗ basis set. UQCISD(T) calculations were performed on the H+CH3→CH4 reaction system with both the 6‐31G∗∗ and 6‐311++G(3df,3pd) basis sets to calibrate the accuracy of the 6‐31G∗∗ basis set results for the H‐atom plus constrained tert‐butyl radical. The above information was used to construct an analytic potential energy function for H‐atom association with a radical site on the (111) surface of diamond, which was then employed in a canonical variational transition state theory (CVTST) calculation of the association rate constant. The resulting rate constant is 1.8-2.1×1013 cm3 mol−1 s−1 for the 1000-2000 K temperature range. It is insensitive to the gas‐phase H‐atom/surface H‐atom nonbonded potential and the potential for the diamond lattice. The H+diamond (111) CVTST rate constant is used to estimate a rate constant of 4×1013 cm3 mol−1 s−1 for H+tert‐butyl association at 298 K. The UQCISD(T)/6‐31G∗ calculations give a H--C(CH3)3 bond dissociation energy which is only 1 kcal/mol lower than the experimental value.