  We present a comprehensive a priori error analysis of a practical energy
based atomistic/continuum coupling method (Shapeev, arXiv:1010.0512) in two
dimensions, for finite-range pair-potential interactions, in the presence of
vacancy defects.
  The majority of the work is devoted to the analysis of consistency and
stability of the method. These yield a priori error estimates in the H1-norm
and the energy, which depend on the mesh size and the "smoothness" of the
atomistic solution in the continuum region. Based on these error estimates, we
present heuristics for an optimal choice of the atomistic region and the finite
element mesh, which yields convergence rates in terms of the number of degrees
of freedom. The analytical predictions are supported by extensive numerical
tests.
