I am studying Pd(II) metal-organic systems and have used GFN2-xTB (with implicit solvation) for high-throughput optimisation of candidate structures. I use the relative energies (from xtb) of different structural isomers as a screening metric based on empirical results. For the sake of validation, I aim to recreate those relative isomer energy calculations using DFT (optimisation, also with implicit solvation), which has previously been shown to agree with the xtb relative energies, for the top candidates.
I am aware that GFN2-xTB is parameterised with a focus on geometries, frequencies and NCIs - not energies, so validation (by DFT) of its use in my workflow is important.
I have a single case so far (although there may be more) where the relative isomer energies from DFT (PBE0/def2-svp/D3BJ) and xTB are opposing and in this case, I expect the difference to be due to a specific chemical functionality.
My question here is about the approaches one might use to understand this type of result. I do not have a lot of expertise in exploring the specific components of DFT/semiempirical calculations. So far, I have tried to understand this effect with a small model system of the different isomers that isolates the effect of the troublesome functionality. However, I am curious about the best way to analyse the results. I.e. is comparing the energy difference of all of the components (electronic energy, Gsolv, dispersion correction) appropriate?
If anyone could point me in the direction of a good resource on this type of issue, that would be appreciated. I am currently in the process of going back to some basic DFT reading.