Triatomic molecules, radicals and ions: how accurate can ab initio calculations be?

The XPO molecules, with X=H,F,Cl,Br, the HCS and HSC radicals and the HCS+ and HSC+ cations have been investigated in order to assess the accuracy that ab initio calculations can reach in evaluating structural and molecular properties. To this purpose, the coupled cluster method in conjunction with correlation consistent basis sets ranging in size from triple to sextuple zeta has been employed. In order to obtain very accurate results, after extrapolation to the complete basis set limit, additional corrections due to core-valence correlation effects have also been included.