# Calculating bond disassociation energy and bond energy for ATP [closed]

I'd like to calculate Bond Dissociation Energy and Bond Energy for ATP to a similar molecule. What free (at least for academic use) computer programs which natively support and document BDE?

If you don't mind, could you provide a tutorial for using those programs to perform the calculations?

• I think your other question goes into this and more, so I'm closing this one for the time being. – jonsca May 24 '13 at 3:33

An example program is GAMESS. It is free. But be warned: the calculation of BDEs is an extremely difficult thing to do. I am presuming that you are a beginner, so you have to be careful for issues surrounding which BDE: \begin{align} \ce{A-B &-> A+ + B-} \tag{ionic}\\ \ce{A-B &-> A. + .B} \tag{homolytic} \end{align}

Most electronic structure theory programs assume gas-phase, absence of solvent. We know that homolytic dissociation is favored in the gas phase, as charge separation into ions is unfavorable in a Coulombic energy.

That being said, the run-of-the-mill electronic structure theory calculations (e.g., HF, DFT, MP2, CCSD, etc. etc.) will not break the atoms apart in a homolytic manner. They tend to break apart atoms in the ionic fashion. But this is a problem, as ions are abhorrent in the gas phase! Basically, simple electronic structure theory methods over-estimate the bond strength, as the product of the dissociation (ionic) is erroneous.

You must use multiconfigurational methods to get it right (e.g. CASSCF, MCSCF). But these are very, very difficult unless you have had a lot of training. I did them for years, and they were still hard for me. But they will give you $\ce{A. + .B}$ as a product! That is the gas-phase BDE.

You may be able to study closed-shell, closed-shell dissociations without much problem, using the HF and DFT methods. Examples include dimers, host-guest interactions where the products look like good-old fashioned molecules that can stand by themselves. (Think water dimers, etc.)

Also, beware of Solvent Effects (favoring ionic dissociation $\ce{A-B -> A+ + B-}$) and solvation energy. In fact, the solvation energy is why $\ce{Na-Cl}$ breaks apart into $\ce{Na+}$ and $\ce{Cl-}$. In the gas phase, it would be $\ce{Na.}$ and $\ce{Cl.}$ radicals.

In summary, BDEs are very difficult things to compute to qualitative accuracy. Quantitative accuracy is even harder. You may want to have an expert look over your procedure and results before you trust them completely.

• Ah yes, bond breaking with electronic structure theory... what a massive headache. – LordStryker Sep 19 '14 at 21:33