Preliminary note

TURBOMOLE offers three programs for MP2 energy and gradient calculations. A "conventional" implementation [116], mpgrad, based on the calculation of four-center integrals, an implementations which uses the resolution-of-the-identity (RI) approximation [117] as part the RI-CC2 program [8], ricc2, and an implementation which is meant for very large (> 100 atoms and > 3000 basis functions) single point MP2 energy calculations. The latter program used in addition to a local RI also a hybrid OSV-PNO approximation reduce the scaling of the computational costs with the system size to (at most) O(N3) but has for small and medium-sized systems a larger prefactor than ricc2.