Preliminary note

TURBOMOLE offers three programs for MP2 energy and gradient calculations. A "conventional" implementation [240], mpgrad, based on the calculation of four-center integrals, an implementation which uses the resolution-of-the-identity (RI) approximation[241] as part the RI-CC2 program[12], ricc2, and an implementation which is meant for very large (\(> 100\) atoms and \(> 3000\) basis functions) single point MP2 energy calculations in the pnoccsd program. The latter program uses in addition to a local RI also a hybrid OSV-PNO approximation to reduce the scaling of the computational costs with the system size to \(\approx {\cal O}({\cal N}^2)\) but has for small and medium-sized systems a larger prefactor than ricc2.