What is a good method and basis set for a system that contains heavy atoms (Zn2+ or Co2+) and a hydroxide anion? The other atoms in the system are phosphorus, sulfur, chlorine, and a few amino acid side chains. As I understand it, the anion requires a "diffuse" function, but I’m not sure if I should apply a different basis to different atoms or treat everything with the same basis. What might be best for a system like this?
I'd love to hear suggestions from scientists with quantum chemistry experience.
It strongly depends on the 'property' you would like to calculate. Some general comments can be the following; Zn and Co are "heavier" than O or H but I do not think relativistic effects are needed so non-relativistic DFT or other ab-initio methods would still be valid. If you replace Zn with Cd, then relativistic effects might be important for some properties (for example; chemical shifts).
Since both are transition metals, you probably need larger than usual basis sets (TZP or TZ2P in ADF probably work!) and hybrid density functional rather than GGAs.
The effect of diffuse functions, in my experience, can only be known for sure by benchmarking. You need to calculate a desired property with/without diffuse functions and see if there is a drastic change. Usually, large systems with small negative charge (-1 or -2) does not require diffuse functions, whereas small systems might, again depending on the property you want to compute. Hope this helps.
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