  We derive an analytical expression for the solution of a two-dimensional
quasicontinuum method with a planar interface. The expression is used to prove
that the ghost force may lead to a finite size error for the gradient of the
solution. We estimate the width of the interfacial layer induced by the ghost
force is of $\co(\sqrt{\eps}\,)$ with $\eps$ the equilibrium bond length, which
is much wider than that of the one-dimensional problem.
