Abstract. We present coupled cluster ab initio calculations for the Mg–He and Ca–He van der Waals potential energy curves. Our calculations employ large atom-centred basis sets, diffuse interbond functions and a fully iterative treatment of single, double and triple excitations in the coupled cluster calculation, and also include corrections for core–valence correlation effects. The use of bond functions makes computations near the complete basis set limit tractable, while the coupled cluster approach gives potential energies in very good agreement with those obtained from full configuration interaction calculations. We find the Mg–He binding energy to be 5.00 cm⁻¹ at an equilibrium distance of 5.1 Å, while the Ca–He binding energy is 3.43 cm⁻¹ at an equilibrium distance of 5.9 Å. A fully iterative treatment of triple excitations increases the Mg–He binding energy by about 0.3 cm⁻¹, or roughly 6% of the total well depth, when compared with typical perturbative treatments of triple excitations.

Print publication: Issue 14 (28 July 2003)
Received 18 March 2003
Published 7 July 2003