  We introduce a computational approach to estimate the hardness and stiffness
of diamond surfaces and nanoparticles by studying their elastic response to
atomic nanoindentation. Results of our ab initio density functional
calculations explain the observed hardness differences between different
diamond surfaces and suggest bond stiffening in bare and hydrogenated fragments
of cubic diamond and lonsdaleite. The increase in hardness and stiffness can be
traced back to bond length reduction especially in bare nanoscale diamond
clusters, a result of compression that is driven by the dominant role of the
surface tension.
