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 higher) is recommended. When using SVP basis sets a qualitative trend can be expected at the most. Basis sets much larger than TZVPP usually do not significantly improve geometries of bonded systems, but still can improve the energetic description. For non-bonded systems larger basis sets (especially, with more diffuse functions) are needed.
• It is recommended to exclude all non-valence orbitals from MP2 calculations, as neither the TURBOMOLE standard basis sets SVP, TZVPP, and QZVPP nor the cc-pVXZ basis set families (with X=D,T,Q,5,6) 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 Define (orbitals below -3 a.u. are frozen) provides a reasonable guess. If core orbitals are included in the correlation treatment, it is recommended to use basis sets with additional tight correlation functions as e.g. the cc-pwCVXZ and cc-pCVXZ basis set families.
• RI-MP2: We strongly recommend the use of auxiliary basis sets optimized for the corresponding (MO) basis sets.