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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?

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    $\begingroup$ The proper nomenclature is called cis vs trans. See for example cis-platin. Although not clear, it seems your question is asking if the trans conformer will have any hydrogen bonding in the trans conformation. The answer would then be clearly no. $\endgroup$
    – Cody Aldaz
    Apr 8, 2020 at 23:03
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    $\begingroup$ Therefore, you should be able to easily use all types of DFT and basis set combinations that you wish. If you're worried about long-range interactions consider long-range corrected functionals, and dispersion. Dispersion is practically free so it's usually a good idea to include it. $\endgroup$
    – Cody Aldaz
    Apr 8, 2020 at 23:05
  • $\begingroup$ Thank you, Cody. Trying to make clear my question, I'm not interested just in this particular compound. The square planar complex example has only two possible configurations (cis/trans), what makes it trivial. But in complexes with higher number and a more diverse set of ligands, it's not trivial. I'm interested in a fast method to ordering a large number of such combinations by relative strength of the interligand interactions. $\endgroup$
    – ksousa
    Apr 10, 2020 at 18:21
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    $\begingroup$ You don’t tell anything about your available ressources, but if you have the capacities, go for something like Tight-DLPNO-CCSD(T)/def2-QZVPPD//TPSS-D3(BJ)/def2-TZVP. $\endgroup$
    – pH13 - Yet another Philipp
    Apr 10, 2020 at 23:23
  • $\begingroup$ The resources are small-scale. Desktop machine, 8 cores/16GB RAM. $\endgroup$
    – ksousa
    Apr 10, 2020 at 23:30

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