### 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) (^{3}) but has for small and medium-sized systems a larger
prefactor than ricc2.