  We show how to map a given n-qubit target Hamiltonian with bounded-strength
k-body interactions onto a simulator Hamiltonian with two-body interactions,
such that the ground-state energy of the target and the simulator Hamiltonians
are the same up to an extensive error O(epsilon n) for arbitrary small epsilon.
The strength of interactions in the simulator Hamiltonian depends on epsilon
and k but does not depend on n. We accomplish this reduction using a new way of
deriving an effective low-energy Hamiltonian which relies on the
Schrieffer-Wolff transformation of many-body physics.
