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As per my understanding, in molecular mechanics (MM) the computed strain energy is based off the strain inherent in the molecule under study relative to an idealized form. For example, cyclobutane to its acyclic form.

However, what about computed values from other methods like PM6 energies, HF energies or MP2 enthalpies. What are these computed values relative to?

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  • $\begingroup$ At least in HF and post-HF, energies are absolute. $\endgroup$
    – pH13 - Yet another Philipp
    Apr 12, 2018 at 21:11
  • $\begingroup$ @pH13-YetanotherPhilipp are they? ADF for example does relative to isolated atoms (or fragments) and Gaussian does relative to isolated nuclei and electrons iirc. $\endgroup$
    – DSVA
    Apr 12, 2018 at 21:54
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    $\begingroup$ "relative to isolated nuclei and electrons" is the same thing as an absolute (electronic) energy though, isn't it? $\endgroup$
    – pentavalentcarbon
    Apr 12, 2018 at 22:12

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Ab initio calculations give the absolute energy of the system of fixed nuclei and moving electrons (all considerations on the type of calculations are implicit such as the Born-Oppenheimer approx.). There isn't any distinction of concepts of absolute energies between softwares when ab initio methods are employed (no ambiguity is allowed as pointed out by Pentavalentcarbon). Maybe the confusion can be illustrated with an example: you can't obtain heats of formation, as an absolute energy, from an ab initio calculation but it serves as a good approximation depending on the method.

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    $\begingroup$ 'There isn't any distinction of concepts of absolute energies between softwares when ab initio methods are employed" is what I said, but you need to be careful considering that the comment about ADF is correct, and MOPAC (AM1, PM3, PM6, PM7, ...) typically print heats of formation. $\endgroup$
    – pentavalentcarbon
    Apr 15, 2018 at 3:28
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    $\begingroup$ "you can't obtain heats of formation, as an absolute energy, from an ab initio calculation' is somewhere between untrue and unclear. It is pretty straightforward to calculate a heat of formation from a series of a calculations. My confusion here is what you mean by "as an absolute energy". It's true that it won't be exact and is only as good as your model, but I feel like you're saying something else. $\endgroup$
    – pentavalentcarbon
    Apr 15, 2018 at 3:31
  • $\begingroup$ First of all the concept of "absolute energy" seemed too loose. The concept that I adopted to try to answer the question is that it is not a quantity obtained as the total energy but you need successive calculations to calculate the heat of formation. Moreover, for other properties these additional steps may be approximations on top of the central ab initio method which is an approximation as well. $\endgroup$
    – rbw
    Apr 17, 2018 at 12:06
  • $\begingroup$ I agree 100% with your comment. I see what you're trying to do, but ultimately it doesn't answer the question. It isn't about how many steps are required to calculate some kind of energy, it's asking what the energy is that you get, and what the energy is relative to. $\endgroup$
    – pentavalentcarbon
    Apr 17, 2018 at 22:14

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