Relativistic configuration interaction calculations for polyatomics: Applications to PbH2, SnH2, and GeH2

A relativistic configuration interaction scheme is described for polyatomic molecules containing heavy atoms. In this method first complete active space MCSCF followed by large scale configuration interaction calculations are carried out. The natural orbitals generated in the large scale CI are then used in the relativistic CI (RCI) calculations, which include the spin–orbit integrals. The spin–orbit integrals are obtained using large Gaussian basis sets, with the operator expressed as a difference of relativistic effective core potentials, and then transformed over the natural orbitals. The transformed integrals are included as one-electron matrix elements in the RCI. This procedure thus takes into account both electron correlation and spin–orbit effects. The method is applied to the spin–orbit states derived from the three low-lying states of PbH2, SnH2, and GeH2 (1A1, 3B1, and 1B1). The spin–orbit mixings of the 1A1 and 3B1(A1) states in the RCI wave functions of PbH2 and SnH2 were found to be quite significant. For PbH2 this mixing lowers the 1A1 state by 1308 cm−1 while the 3B1(A1) is raised by 1371 cm−1 with respect to the 3B1 state without the spin–orbit splitting. The dipole moments of all the three radicals reveal that PbH2 has the largest dipole moment of the three. The electronic states of GeH2 and SnH2 are similar [1A1(A1)−3B1(A1):Te=23.1 kcal/mol for GeH2, Te=23.8 kcal/mol for SnH2], but the electronic states of PbH2 differ [1A1(A1)–3B1(A1):Te=41 kcal/mol] from their lighter analogs in that the splittings of the excited states are considerably higher. The Mulliken population analyses of the CI natural orbitals reveal that this is primarily a result of the relativistic stabilization of the 6s orbital of the lead atom due to the mass–velocity contraction. The geometries of the A1 component of the 3B1 state are altered to a considerable extent by the spin–orbit term for both PbH2 and SnH2.