Vdw energy calculus

Dear all,

I have some problems using Avogadro. I was doing some calculus of metal-organic complexes and I saw that the energy was too high specially due to Van der Waals. I saw that the problem is in aromatic systems so I tried with the benzene and I have the same problem: a too high energy caused by Van der Waals effects. All these calculus I have done it with UFF but I tried MMFF94 and there is also the problem… Does anyone know how to solve this?

Moreover, I observed that if you have two benzenes bonded, in UFF the two rings remain in the same plane. Is this normal? In my opinion they should be in different planes.

Thank you very much.

The force fields you mention are empirical. They have reasonable, rough approximations and are fairly good at producing geometries.

I would not trust the energies from them. (Indeed, we just published a paper on this.)

I’m not sure about your question on UFF. It sounds like you’re asking if the benzene dimer should be co-planar? Again, force fields include approximations. Most do not handle π-π interactions well.

If you want accurate energies of metal-organic complexes or geometries of π-π interactions, you need to use quantum chemical methods.