10

Because the hydrogenation release has a barrier (activation energy) that is not too low. The fact that the dehydrogenation energy of $\ce{H3N-BH3}$ is negative does prove that it will eventually decompose if left at room temperature, but it does so extremely slowly so that you don't notice, and it's more useful to treat it as stable. It's like, your ...


7

I'm surprised that a single geometry step could result in a converged structure that's 15 kcal/mol lower in energy. That is odd, although not impossible. I would recommend carrying out a frequency calculation on the structure. It should, of course, have no imaginary modes. I'd also consider running a "regular" geometry optimization on the resulting ...


3

Double hybrids contain MP2 so the basis set dependence is similar to MP2; the only thing you gain is the overall weighting factor that is used to damp the MP2 correction. The reason to use double hybrids is just to get results of better than MP2 quality at a cost similar to MP2. But, in many cases modern range-separated functionals like $\omega$B97M-V out-...


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