An experimental colleague asked me how hard it would be to calculate homolytic bond-dissociation energies for different phosphonates which are involved in a Hydrophosphination. The compounds include aryl and alkyl phosphonates (R2P=O), and the oxygen can be replaced with BH3 (R2P-BH3).

They just want a trend, so the absolute value doesn't matter as long as the model is useful for getting relative trends correct. Blackbox methods like DFT are preferred. Maybe I'm overthinking this but isn't it difficult to calculate BDE because at the dissociation limit the $\sigma$ and $\sigma^*$ become degenerate, and more than one electronic configuration can describe the system (multi-configurational), e.g. $(\sigma)^2$, $(\sigma^*)^2$? Therefore, the correct wavefunction should be sufficiently flexible to treat both configurations on the same footing, which single-references like Hartree-Fock, and DFT cannot. However, multi-configuration techniques like CASPT2/CASSCF are complicated and I'd probably not be able to easily instruct a colleague how to use them easily.

Besides CASPT2/CASSCF, what other methods for calculating homolytic BDEs are good and are fairly "black-box"?

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    $\begingroup$ To calculate a dissociation energy, don't you just need the energy at the equilibrium, and then the energies of the constituents into which the system dissociates? The problem of near-degenercies, for example near the dissociation of the N$_2$ molecule, means one needs a multi-reference method to calculate the potential energy surface, but a single-reference method should be fine for the N$_2$ dissociation energy right? A single-reference method is fine for the equilibrium of N$_2$ and and fine for the N atoms into which N$_2$ dissociates. What molecule do they want to simulate? $\endgroup$ – Nike Dattani May 23 at 1:29
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    $\begingroup$ The simplest case of an aryl R$_2$P-H would be something with two aromatic rings and P-H? Minimum of 12 carbon atoms + 24H + P + H, meaning 38 atoms? I think it would not hurt to ask the experimental colleague how many atoms, but I have a rough idea of how to answer. $\endgroup$ – Nike Dattani May 23 at 2:03
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    $\begingroup$ How exact of an answer do they want? $\endgroup$ – kskinnerx16 May 24 at 20:15
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    $\begingroup$ If this is the case, more simple DFT methods can be used. Unless I am interpreting the question incorrectly. However, a DLPNO CCSD(T) cc-pVTZ combination is probably best for that system size. $\endgroup$ – kskinnerx16 May 25 at 6:12
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    $\begingroup$ @Greg It is definitely overkill. Personally I like using B3LYP 6-311++G**. My comment was also made before OP added the DFT part of the question. $\endgroup$ – kskinnerx16 yesterday

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