The molecular structure of 1-oxo-1,2,4,3,5-trithiadiazole (1) has been studied theoretically by a large number of computational methods. The results of calculations using different DFT functionals are compared with those obtained from HF, MP2, MP4(SDQ), QCISD and CCSD calculations. In addition, the influence of a number of basis sets on the calculated geometry was investigated. The results of the post-HF methods show that improving the level of theory from CCSD to CCSD(T) still has a considerable effect on the geometry. Taking the CCSD/cc-pVTZ geometry as a reference the DFT/B1B95/aug-cc-pVTZ combination appears to provide the most valuable description of all the examined functionals. (c) 2006 Elsevier B.V. All rights reserved.