Ab initio potential-energy curves for the molecular ions NeH⁺ and ArH⁺

Potential energy curves for the X¹∑⁺ ground states of the molecular ions NeH⁺ and ArH⁺ have been obtained from ab initio calculations in which large basis sets were used. Electron correlation was taken into account by the configuration interaction, MP2 and coupled-cluster methods. In order to get satisfactory agreement with experimental spectroscopic data it was found to be important to employ a size-consistent method for the treatment of electron correlation and to include a correction for the basis set superposition error. In the best calculations, transition wavenumbers are obtained within 0·3% of the experimental values.