  Generalized relativistic effective core potential (GRECP) calculations of
spectroscopic constants of the HgH molecule ground and low excited states and
the HgH+ cation ground state are carried out, with correlation included by the
Fock-space relativistic coupled cluster (RCC) method. Basis set superposition
errors (BSSE) are estimated and discussed. It is demonstrated that connected
triple excitations of the 13 outermost electrons are necessary to obtain
accurate results for mercury hydride. Spectroscopic constants derived from
potential curves which include these terms are in very good agreement with
experiment, with errors of a few mbohr in Re, tens of wavenumbers in excitation
energies and vibrational frequencies, and proportionately for other properties.
Comparison with previous calculations is also presented.
