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wham09

Member

Registered: 2025-09-19

Posts: 57

Basis set requirement for anions under very high level of theory

Dear Prof. Lu,

I wonder if diffuse functions are "always" required for reaction energy calculation of anions, even if the calculation level is already very high. For example, in the following 4 scenarios, should diffuse functions be added?

1. ordinary hybrid functional (e.g. PBE0-D4 or wB97M-V) with def2-QZVPP: Should change to ma-QZVPP?
2. double hybrid functional (e.g. PWPB95-D4 or wB97M(2)) with def2-QZVPP: Should change to ma-QZVPP?
3. DLPNO-CCSD(T) with CBS (def2-TZVPP / def2-QZVPP): Should change to ma-TZVPP -> ma-QZVPP?
4. DLPNO-CCSD(T) with CBS (cc-pVTZ / cc-pVQZ): Should change to aug-cc-pVTZ -> aug-cc-pVQZ?

sobereva

Tian Lu (Multiwfn developer)

From: Athens, Greece

Registered: 2017-09-11

Posts: 2,204

Website

Re: Basis set requirement for anions under very high level of theory

The answer depends on if there are atoms with significant negative charge (e.g. ADCH atomic charge < -0.5) and meantime directly participating in the reaction, it is not dependent of theoretical method. Only those atoms need diffuse functions to improve result.
However, note that if basis set extrapolation is used, the necessity of adding diffuse functions is somewhat weakened.

wham09

Member

Registered: 2025-09-19

Posts: 57

Re: Basis set requirement for anions under very high level of theory

Thank you for the reply. To further clarify, if the conditions you explained (significantly negative charge on certain atoms, and their participation in the reaction) are met, addition of diffuse functions to those atoms is required even if the basis set is already QZ or 5Z quality?

Additional: I notice that in the Multiwfn’s orca input generating function, the option for DLPNO-CCSD(T)/CBS (3,4/def2) disappears when I activate the diffuse function option, and only the option for extrapol(3,4/aug-cc) remains. Is this because ma-TZVPP/ma-QZVPP are somewhat less appropriate for extrapolation?