ON THE EXISTENCE OF SH3, SEH3, AND TEH3 - DISCREPANCIES BETWEEN ALL-ELECTRON AND PSEUDOPOTENTIAL CALCULATIONS

Ab initio calculations using both pseudopotential and double and tripl e-zeta all-electron basis sets, with and without electron correlation (MP2, QCISD), have been performed on the lambda(4)-sulfanyl (SH3), lam bda(4)-selanyl (SeH3), and lambda(4)-tellanyl (TeH3) radicals. All-ele ctron basis sets of double-zeta quality predict that SH3 and SeH3 corr espond to transition states on their respective potential energy surfa ces. In contrast, the pseudopotentials of Hay and Wadt predict that SH 3 and SeH3 correspond to local minima at the QCISD level of theory whi le the pseudopotentials of Christiansen and Stevens predict transition states. By comparison, TeH3 proved to be a local minimum at all level s of theory. Interestingly, when a very large (triple-zeta) all-electr on basis set was used, SH3 proved to be a transition state; however, i n this instance the potential energy surface was found to be much flat ter than in the case for which a double-zeta basis set was used, sugge sting that further improvements in the basis set may lead to a local m inimum. Further improvements in the all-electron selenium basis also l ed to a local minimum for SeH3 at the QCISD level of theory. (C) 1995 by John Wiley & Sons, Inc.