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Re: FOOF torsional potential


Fluorine is known to be a strongly coorelated system, so, methods without correlation are not recommended at all. In addition, MP3 frequently performs worse than MP2, while MP4 restores more or less correct values.

On Wed Oct 3 '12 12:25pm, Alexey Nikitin wrote
>Hi all

>I had a problem with the calculation of the torsion potential in the molecule FOOF.
>When rotating the torsion angle two strange effects observed:
>1.     F-O bond length is changed up to 0.2 A
>2.     The energy barrier of rotation differs by almost 100 kcal / mol (!!!), depending on the method: HF, MP2, MP3, MP4. Moreover MP3 worse than MP2.

>(I used the basis sets: 6-311+G(2d,p), cc-pVDZ and cc-pVQZ)

>Could you be so kind to explain what the problem is and how to overcome it?
>And more general question. I still considering the calculation of torsion angles in the cc-pVQZ MP2 as a very robust procedure. When it does not work?

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