To model the first bond dissociation energy of a reaction like:
MX6 --> MX5 + X
I understand HF is not a suitable level of theory as it cannot describe bond dissociation due to its inability to account for electron correlation.
How suitable would PM6, CCSD (T) and B3LYP be?
Dear Tanri Vak,
If your electronic system is not too big (many atoms) I would recommend to use CCSD (T) or MP2. Since both methods use a single determinant wavefunction (HF), they give reasonable energies and geometries by taking the dynamical correlation into account. Moreover, these methods are size-consistent which means that you can calculate the dissociation energy by taking the difference of products and reactants. B3LYP is a DFT based approach involving a hybrid exchange-correlation functional using empirical parameters whereas PM3 is a semi-empirical method which should not give more accurate results than the post-HF methods explained above. As aforementioned, it is a matter of computational costs and I did not dive into details by explaining e.g. the expression (T) being a perturbative correction for the triple substitutions but I can recommend the book by A. Szabo and N. S. Ostlund "Modern Quantum Chemistry" introducing the reader to these different methods by providing good examples.
Hope this helps!
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