The torsional barrier of glyoxal has been extensively studied using state-of-the-art ab initio methodologies ranging from Hartree-Fock (HF) and Moller-Plesset theories up to several developments of the coupled-cluster (CC) Formalism, namely: CCD, CCSD, CCSD(T), ED and BD(T). We present a thorough discussion about the influence of the basis set size on the results, as well as a comparison between all the theories employed. The values obtained From the CCSD or CCSD(T) treatment achieve the so-called 'calibration accuracy' (+/-1 kJ/mol) from which an extremely ac curate torsional potential has been derived.