COMMENT ON THEORETICAL CALCULATIONS OF THE NUCLEAR MAGNETIC SHIELDINGTENSORS FOR THE ETHYLENIC CARBON-ATOMS IN CYCLOPROPENES

We compare several methods for the calculation of NMR shielding tensor s at the Hartree Fock level (most of them using some sort of distribut ed gauge origins), applied to the C-13 shielding tensors in cycloprope ne C3H4 and its dichloro and tetrachloro derivatives, C3H2Cl2 and C3Cl 4. Among these methods is one recently proposed by Smith et al. that i n our opinion should not be called IGLO. Since multiconfiguration IGLO calculations indicate that correlation effects are important in these molecules, the quality of Hartree-Fock based methods cannot be judged from a comparison with experiment. Instead a comparison with the Hart ree-Fock limit is necessary. Therefore IGLO benchmark calculations for these molecules using large basis sets are presented.