A full CI treatment of the 1A1, 1B1, and 3B1 states of SiH2

Full CI calculations are presented for the 1A1, 3B1, and 1B1 states of SiH2 at their respective equilibrium geometries and at geometries with the SiH bonds stretched. These results are compared with those obtained from single-reference and multireference CI calculations. The computed Te values agree well with the full CI results, provided that the effects of higher-than-double excitations are accounted for either by the Davidson correction or by a multireference approach. When the SiH bonds are stretched, the single-reference methods are not sufficiently flexible, and only CASSCF/MRCI achieves chemical accuracy (i.e., agrees with the full CI to 1 kcal/mol). Overall, the accuracy of the various approximate methods is very similar to that found for H2O, NH2, and CH2.