The extremely small (around 0.80 kcal/mol) barrier for the -SH rotation of thiophenol is studied by using MP2, CASSCF(12,11), and three DFT (B3LYP, HCTH, and omega B97X-D) methods with various basis sets. The 6-311++G(3df,3pd) turns out to be the proper minimal basis-set for reliable calculations. CCSD(T) and CASPT2//CASSCF calculations confirm that the planar conformation of the -SH is the minimum-energy structure of not only the ground So but also the first excited S-1 electronic states. The calculated torsional barrier of the S-1, 1.89 kcal/mol, is also quite small. (C) 2013 Elsevier B.V. All rights reserved.