Rotational-tunneling transitions including quadrupole substructure in (HBr)-Br-79-(HBr)-Br-79, (HBr)-Br-79-(HBr)-Br-81, and (HBr)-Br-81-(HBr)-Br-81 have been recorded and assigned. When combined with previously recorded rovibrational infrared and microwave data, this provides a basis for optimizing a ground state four-dimensional morphed potential with quantifiable uncertainties. The determined potential minimum has a value V = -644.0 cm(-1) at the geometry R = 4.04 &ANGS;, θ(1) = 16.9°, θ(2) = 108.3°, and &UPhi; = 180.0°. The peak of the minimum energy path between the two equivalent minima occurs at V = -571.2 cm(-1) leading to a barrier height of 72.8 cm(-1). © 2005 Elsevier B.V. All rights reserved.