  The generalized relativistic effective core potential (GRECP) approach is
employed in the framework of multireference single- and double-excitation
configuration interaction (MRD-CI) method to calculate the spin-orbit (SO)
splitting in the $^2P^o$ ground state of the Tl atom and spectroscopic
constants for the $0^+$ ground state of TlH. The 21-electron GRECP for Tl is
used and the outer core 5s and 5p pseudospinors are frozen with the help of the
level shift technique. The spin-orbit selection scheme with respect to
relativistic multireference states and the corresponding code are developed and
applied 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 optimized for
the Tl atom and employed in the TlH calculations. Very good agreement is found
for the equilibrium distance, vibrational frequency, and dissociation energy of
the TlH ground state ($R_e=1.870 \AA, \omega_e=1420 cm^{-1}, D_e=2.049 eV$) as
compared with the experimental data ($R_e=1.868 \AA, \omega_e=1391 cm^{-1},
D_e=2.06 eV$).
