Tunneling splittings in the water dimer have been determined by the semiclassical WKB method, based on pathways characterized at the computational level of second-order Moller-Plesset (MP2) theory with basis sets of aug-cc-pVTZ quality. This calculation takes into account all three acceptor tunneling, donor-acceptor interchange, and bifurcation tunneling rearrangements of the water dimer. The tunneling splittings were evaluated as 7.73 cm(-1) (large splitting) and 0.42 cm(-1) (small splitting), which are in good agreement with the corresponding experimental values of 11.18 cm(-1) and 0.70 cm(-1), respectively. (C) 2004 American Institute of Physics.