Generalized relativistic effective core potential and relativistic coupledcluster calculation of the spectroscopic constants for the HgH molecule and its cation

Generalized relativistic effective core potential (GRECP) calculations of s pectroscopic constants of the HgH molecule ground and low excited states an d the HgH+ cation ground state are carried out, with correlation included b y the Fock-space relativistic coupled cluster (RCC) method. Basis set super position errors (BSSE) are estimated and discussed. It is demonstrated that connected triple excitations of the 13 outermost electrons are necessary t o obtain accurate results for mercury hydride. Spectroscopic constants deri ved from potential curves which include these terms are in very good agreem ent with experiment, with errors of a few mbohr in R-e, tens of wave number s in excitation energies and vibrational frequencies, and proportionately f or other properties. Comparison with previous calculations is also presente d. (C) 2001 American Institute of Physics.