Wave functions with terms linear in the interelectronic coordinates to take care of the correlation cusp. III. Second‐order Mo/ller–Plesset (MP2‐R12) calculations on molecules of first row atoms

The MP2‐R12 method (Mo/ller–Plesset second‐order perturbation theory with terms linear in the interelectronic coordinate r₁₂) in the approximations A and B as outlined in paper I of this series is applied to the ground states of the molecules H₂, LiH, HF, H₂O, NH₃, CH₄, Be₂, N₂, F₂, C₂H₂, and CuH in their experimental equilibrium geometry, and to the van der Waals interaction between two He atoms. In all cases MP2 correlation energies are obtained that are supposed to differ by at most a few percent from the basis set limit. For CH₄ the dependence of the energy on the symmetric stretching coordinate is studied, which together with other information leads to a recommended bond length of 1.086 Å for the CH bond length. For He₂ and F₂ the canonical and localized descriptions are compared. The latter is superior for the K‐shell contributions, otherwise there is a little difference. For He₂ in the localized representation rather good results for the dispersion interaction are obtained. The potential curve of Be₂ is significantly improved in MP2‐R12 as compared to conventional MP2. The examples C₂H₂ and CuH show that the method is not limited to very small systems.