  A continuous-time random walk in the quarter plane with homogeneous
transition rates is considered. Given a non-negative reward function on the
state space, we are interested in the expected stationary performance. Since a
direct derivation of the stationary probability distribution is not available
in general, the performance is approximated by a perturbed random walk, whose
transition rates on the boundaries are changed such that its stationary
probability distribution is known in closed form.
  A perturbed random walk for which the stationary distribution is a sum of
geometric terms is considered and the perturbed transition rates are allowed to
be inhomogeneous. It is demonstrated that such rates can be constructed for any
sum of geometric terms that satisfies the balance equations in the interior of
the state space. The inhomogeneous transitions relax the pairwise-coupled
structure on these geometric terms that would be imposed if only homogeneous
transitions are used.
  An explicit expression for the approximation error bound is obtained using
the Markov reward approach, which does not depend on the values of the
inhomogeneous rates but only on the parameters of the geometric terms.
Numerical experiments indicate that inhomogeneous perturbation can give smaller
error bounds than homogeneous perturbation.
