I'm calculating charge transfer complex (donor-accepter intermolecular system) by density functional theory. Since the functional has a van der Waals term, I'm rotating the acceptor and checking the molecular interaction.
Strangely, I could find regions where the intermolecular distance increases with decreasing binding energy, despite the larger charge transfer degree. Up to this point, I thought that van der Waals interaction(binding energy) was stronger than charge transfer. However, in the region where the binding energy and the CT degree decrease together, I could find that the intermolecular distance also decreases in certain region. The CT degree was calculated by the variation of bond distance in the acceptor.
Is it physically possible? What should I consider more in order to explain this region?
Actually, the information is not so sufficient to give an adequate answer.
In my opinion, if you did computation of dihedral angle dependent intermolecular interaction energy, the interrelationship between interaction energy and distance can be incoherent. Because the repulsive interaction (by steric hindrance) importantly contribute to the binding energy.
However, prior to accept above interpretation,
1) you may correct the binding energy with BSSE calculation.
2) try to use other version of vdW with proper keywords.
3) try to use other DFT functional such as double-hybrid functional.
In DFT the binding energy in a dimer is an empirical value found by the method of molecular mechanics, where the interactions between all atoms are summed up. Perhaps with an increase in the distance between specific atoms of monomers, the sum of the distances between all atoms in the dimer decreased
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