The molecular structure of the title compound, 5-oxo-1,3,2,4-dithiadiazole or Roesky's ketone, is studied theoretically by HF, DFT and MP2 calculations and the results are compared to the QCISD geometry. Within DFT the structure is calculated using an elaborate series of functionals. A number of extended basis sets were used to investigate their influence on the calculated geometry. Of the nineteen evaluated method/basis set combinations none can reproduce the complete QCISD geometry: methods that reproduce the geometry of the NSNS fragment, overestimate the CS bond length and vice versa. No systematic way to improve the overall geometry of Roesky's ketone is apparent from the data. (C) 2004 Elsevier B.V. All rights reserved.