RI-MP2: optimized auxiliary basis sets and demonstration of efficiency

Applications of the RI-MP2 method require high-quality auxiliary basis sets employed to approximate charge distributions. A variational procedure is proposed and applied to optimize auxiliary bases for main group and transition metal atoms which are tested for more than 350 molecules. The RI approximation affects molecular MP2 energies by less than 60 μEh per atom and equilibrium distances by less than 0.2 pm. We further comment on the extension from RHF to UHF and the exploitation of molecular symmetry. Applications to (Cu2S)n clusters and hydrocarbons CnH2n+2 document a significant reduction of computation times which allows for calculations with up to 1000 basis functions in C1 symmetry.