GRECP/MRD-CI calculations of spin-orbit splitting in ground state of TI and of spectroscopic properties of TIH

The generalized relativistic effective core potential (GRECP) approach is e mployed in the framework of multireference single- and double-excitation co nfiguration interaction (MRD-CI) method to calculate the spin-orbit splitti ng in the P-2(0) ground state of the Tl atom and spectroscopic constants fo r the 0(+) ground state of TlH. The 21-electron GRECP for Tl is used, and t he outer core 5s and 5p pseudospinors are frozen with the help of the level shift technique. The spin-orbit selection scheme with respect to relativis tic multireference states and the corresponding code are developed and appl ied in the calculations. In this procedure both correlation and spin-orbit interactions are taken into account. A [4, 4, 4, 3, 2] basis set is optimiz ed for the Tl atom and employed in the TlH calculations. Very good agreemen t is found for the equilibrium distance, vibrational frequency, and dissoci ation energy of the TlH ground state (R-e = 1.870 Angstrom, omega (e) = 142 0 cm(-1) D-e = 2.049 eV) as compared with the experimental data (R-e = 1.87 2 Angstrom, omega (e) = 1391 cm(-1) D-e = 2.06 eV). (C) 2001 John Wiley & S ons, Inc.