Gaussian basis sets orbital energy calculations using

There exists a wide selection of exponents for Gaussian basis sets for nonrelativistic calculations, although most of these are for lighter elements which for most purposes do not require a relativistic treatment. For four-component relativistic calculations, nonrelativistic basis sets can be used for lighter atoms, but as the relativistic effects of orbital contraction and spin-orbit splitting increase in importance, these nonrelativistic basis sets become inadequate. In some measure the orbital contraction for inner orbitals is counteracted by the use of a finite nucleus, which tends to push out the inner parts of the spinors. A major concern is the 2/ i/2 space (Matsuoka and Okada 1989) due to the 5-character of the small component at least two extra functions relative to the nonrelativistic basis are needed for the 6/ block to reduce the error in the energy to 0.5 h-... 

Das and Balasubramanian (1990) computed the spectroscopic constants and potential energy curves of 20 electronic states of LaH. They used RECPs with 5s 5p 5d 6s valence shells for the La atom together with a (5s5p3dlf) valence Gaussian basis set. CASSCF followed by the FOCI method of calculation was used to compute the potential-energy curves while the spectroscopic constants were obtained using the SOCI method. In addition, the spin-orbit effects were introduced using the RCI method. 

---