Ab initio study of the individual interaction energy components in the ground state of the mercury dimer

Relativistic and non-relativistic all-electron, ab initio methods are used to investigate the role of the individual energy components in the total interaction energy involved in the formation of the weakly bound ground state X(1) Sigma(g)(+) of the Hg-2 molecule. The interaction energy is partitioned according to a hybrid approach into a supermolecular repulsive potential and the damped dispersion energy derived from perturbation theory. Both parts are then computed individually and their dependence on relativity and electron correlation investigated. From this analysis, hybrid potentials that comprise different physical interactions are constructed and the importance of specific features is evaluated by comparison of the appropriate hybrid potentials with each other and with the experimental curve. The most detailed model is based on a CASSCF supermolecular potential to which the damped intramonomer correlated dispersion energy series is added. In the relativistic case it yields a potential with quantitatively correct asymptotic behaviour and which is able to predict potential parameters that agree reasonably well with the experimental results. The asymptotic density method developed by Duman and Smirnow was tested as an inexpensive alternative for the calculation of the repulsive part of the hybrid potential. In spite of its recent success in the calculation of the He-He potential, it cannot provide a good repulsive component in the case of Hg-2. As a result of this work, this failure can be attributed to the unusually strong induction effects in the ground state mercury dimer bond, which lower repulsive forces substantially.