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?