The V-6 term, regulating the shape of the internal rotation potential for a single rotor, has been predicted by ab initio theory for acetaldehyde. The internal rotation potential constants were computed by single point fourth order Moller-Plesset (MP4) 6-311G(3df,2p) level theory for three models differing in molecular flexing motions using MP2 optimized molecular geometries. The negative sign of V-6, established by microwave and-infrared experiments, is predicted only by the fully relaxed model which includes out-of-plane wagging of the aldehyde hydrogen. This requires strong coupling of methyl torsional motion to the hydrogen wag. The predicted magnitude of V-6 differs by only 4 cm(-1) from the experimentally established value.