Knowledge Bank

There are a number of ambiguities and curiosities found in the published Si-H stretching vibrational frequencies of the ground states, $X~1A\prime$, and first excited singlet states, $A~1A$, of the halosilylenes HSiF, HSiCl, and HSiBr. Here we have investigated these molecules using self-consistent field. SCF, configuration interaction including all single and double excitations, CISD, and triple excitations, CISDT, and complete active space, CAS-SCF, quantum mechanical methods. For the ground and first excited triplet states, all these methods give similar results, but for the excited singlet states, only the CASSCF results are believable.