AB-INITIO POTENTIAL-ENERGY CURVES AND BINDING-ENERGIES OF AR2 AND MG2

The van der Waals interaction potentials of the argon and magnesium dimers are calculated by the complete fourth-order Moller-Plesset perturbation theory (MP4) with a large basis set incorporated with midbond functions. Preliminary calculations are carried out to test the saturation and stability of the midbond functions with respect to the change in size and position of the midbond function set. It is found that as midbond functions are gradually added to a moderately polarized basis set the calculated interaction energy converges quickly and is highly insensitive to the displacement of midbond functions. Midbond functions are confirmed to reproduce the intersystem correlation energy achieved traditionally by nucleus-centred polarization functions, especially by diffuse polarization functions. The calculated interaction potentials of the two dimers are in good agreement with experiment. For Ar2 the calculation recovers over 92% of the experimental well depth and predicts an equilibrium internuclear distance within 0.1 a0 of experiment. The calculated vibrational frequency is also within 1 cm-1 of experiment. For Mg2 similar accuracy is achieved provided that the effect of core electron correlations is taken into account.