During conformational analysis of 18crown6, it was observed that the conformer with S6 symmetry was having the lowest energy when dispersion correction was not included. However, upon addition of D3BJ dispersion correction, the relative energy ordering changes and the former conformer is no longer having the lowest energy.
Well, 18-crown-6 is a substance that is explicitely meant to have strong interactions. If you now include dispersion correction and thus more interaction, it doesn't sound very surprising to me that the relative stability of conformers may be affected.
Compare the two conformers: Has one of them more polarizable groups in close vicinity? This might explain such differences.
But keep in mind that dispersion effects are caused by electron correlation, and that DFT is handling this rather approximatively. Moreover, a comparison of single-molecule DFT results gives you the energy of a molecule in vacuum. The most stable conformer in solution may be different.
The point by Ulrich Deiters about vacuum vs. solution is very important in this context. In gas phase/vacuum, the potentially binding ether functionalities of 18c6 might search for an internal partner, thus assuming a sort of "odd" structure, while in solution they might relax to a more convenient one since the solvent offers binding partners.
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