  The correlation calculation of the electronic structure of PbH is carried out
with the Generalized Relativistic Effective Core Potential (GRECP) and
MultiReference single- and Double-excitation Configuration Interaction (MRD-CI)
methods. The 22-electron GRECP for Pb is used and the outer core 5s, 5p and 5d
pseudospinors are frozen using the level-shift technique, so only five external
electrons of PbH are correlated. A new configuration selection scheme with
respect to the relativistic multireference states is employed in the framework
of the MRD-CI method. The [6,4,3,2] correlation spin-orbit basis set is
optimized in the coupled cluster calculations on the Pb atom using a recently
proposed procedure, in which functions in the spin-orbital basis set are
generated from calculations of different ionic states of the Pb atom and those
functions are considered optimal which provide the stationary point for some
energy functional. Spectroscopic constants for the two lowest-lying electronic
states of PbH ($^2\Pi_{1/2}, ^2\Pi_{3/2}$) are found to be in good agreement
with the experimental data.
