Recommendation

Use the same basis set type for all atoms; use ECPs beyond Kr since this accounts for scalar relativistic effects.

New basis sets (def2-XYZ): MP2 implies RI-MP2 and RICC2

exploratory

MP2: SVP

almost quantitative

DFT: SV(P), HF: SVP, MP2: TZVPP; properties (HF and DFT): TZVPP

quantitative

DFT: TZVP, HF: TZVPP, MP2: QZVPP

basis set limit

DFT: QZVP, HF: QZVP

If you want a better basis than SV(P), assigned automatically, use b all def2-TZVP (or another basis). The assignment can be checked by bl.

Diffuse functions should only be added if really necessary. E.g. for small anions or treatment of excited states use: TZVP instead of SVP + diffuse. This is more accurate and usually faster. Only for excited states of small molecules or excited states with (a partial) Rydberg character add additional diffuse functions (e.g. by using the aug-cc-pVTZ basis) as well as the keyword diffuse, for more information, see page in the keyword section.

[Old basis sets (def-XYZ): For standard correlated calculations (MP2, RI-MP2, RI-CC2) use the doubly-polarized TZVPP (or def-TZVPP) basis.]