Limiting values for Mo/ller–Plesset second‐order correlation energies of polyatomic systems: A benchmark study on Ne, HF, H₂O, N₂, and He...He

Limiting values for Mo/ller–Plesset second‐order (MP2) correlation energies are provided for the ten‐electron systems Ne, HF, and H₂O, for the N₂ molecule, and for the weak He...He interatomic interaction energy. These limiting values were obtained by the MP2‐R12 approach. This approach differs from traditional MP2 theory by employing first‐order wave functions which explicitly depend on the interelectronic coordinates r_ij. With the MP2‐R12 method, the atomic orbital (AO) basis set limits for the systems under study are reached. The calculations provide insight into AO basis set requirements for methods with linear r_ij dependence (R12 methods), e.g., for coupled cluster methods, or multireference configuration interaction methods. Moreover, it is expected that the results have the potential to serve as valuable benchmarks for further developments in the field of explicitly correlated wave functions, for example for expansions in terms of Gaussian geminals (Gaussian functions which depend on r_ij). The present calculations on HF, H₂O, and N₂ provide the most accurate second‐order correlation energies to date for these systems.