All-electron molecular Dirac–Hartree–Fock calculations: Properties of the XH4 and XH2 molecules and the reaction energy XH4→XH2+H2, X=Si, Ge, Sn, Pb

Abstract

Dirac–Hartree–Fock (DHF) calculations have been carried out on the ground states of the Group IV di- and tetrahydrides. Geometries and infrared data are presented for both sets of molecules, dipole moments for the dihydrides, and the SCF (self-consistent field) reaction energies for the reaction XH4→XH2+H2. The effects of relativity can be seen in shorter bond lengths, higher frequencies for XH4 but lower stretching frequencies for XH2, higher infrared intensities for stretching modes and lower intensities for bending modes, a more positive dipole moment, and decreased stability for XH4 relative to XH2+H2. The results are also compared with relativistic effective core potential (RECP) calculations, first-order perturbation theory (PT) calculations, and with the limited experimental data. PT predicts all properties well for the Ge hydrides, but deviations are noted for the Sn hydrides, which become serious for the Pb hydrides. The full-core RECP calculations generally do not give results as good as PT. The addition of the (n−1)d orbital improves many of the properties. Overall, the quality of the RECPs used is somewhat variable, and no consistent pattern in the deviations from the DHF results is found for the set of RECPs used in this study.