Recommendations

• It is well-known, that perturbation theory yields reliable results only, if the perturbation is small. This is also valid for MP2, which means, that MP2 improves HF results only, if HF already provides a fairly good solution to the problem. If HF fails, e.g. in case of partially filled $d$-shells, MP2 usually will also fail and should not be used in this case.
• MP2 results are known to converge very slowly with increasing basis sets, in particular slowly with increasing $l$-quantum number of the basis set expansion. Thus for reliable results the use of TZVPP basis sets (or comparable) is recommended, when using SV(P) basis sets a qualitative trend can be expected at the most. Basis sets much larger than TZVPP (or cc-pVTZ) usually do not significantly improve results, moreover in this case the errors of the method and those of the basis sets are no longer balanced.
• It is recommended to exclude all non-valence orbitals from MP2 calculations, as neither TURBOMOLE standard basis sets nor cc-pVXZ (X=T,Q,5) are designed for correlation treatment of inner shells (for this purpose polarisation functions for the inner shells are needed). The default selection for frozen core orbitals in RIMP2PREP (orbitals below -3 a.u. are frozen) provides a reasonable guess.
• RIMP2: We strongly recommend the use of auxiliary basis sets optimized for the corresponding (MO) basis sets.