What are the cheapest basis sets able to model intermolecular dipole interactions inside a metal complex and give good relative energies, with DFT?

As a example, If I have a square planar complex of a cation with both a weakly polar ligand (X) and strongly polar ones, like ammonia and water I expect a bit of stabilization due to formation of a hydrogen bond between ammonia and water, if they are in adjacent positions:



```
   NH3
   │
X─ M² ─OH2
   │
   X

```

What would not happen if they were in opposite positions:



```
     X
     │
NH3─ M² ─OH2
     │
     X

```

I think it is a bit tricky to model that, as intermolecular interactions are long-range, and most small basis sets approximations start to break down in such ranges. I imagine it's necessary at least some diffuse and polarization functions to start seeing such interactions. So my guess is that one needs to start at least with 6-31+G(d) level or higher. But I'm not sure. If you need do do a large number of such calculations, would it be possible to go lower than that, and still get reliable results, at least in a relative way, say, finding the first structure has lower energy than the second, even if neither value matches experimental data very well?


