We present results from a series of ab initio calculations that investigate the relative efficiency of tunneling via through-bond and through-space interactions. We use model systems and methods ranging from Hartree-Fock theory to coupled cluster singles and doubles and show that while the through-space coupling decays more rapidly with distance than does through-bond coupling, the prefactor to the exponential decay for through-space coupling is considerably larger than has been previously supposed. The implications of these results are then discussed. (C) 2004 Published by Elsevier B.V.