2
$\begingroup$

can I simply calculate the ionization energy (IE) as the difference between the neutral and the charged system Heat of formation (Hf) with MOPAC ? The same question is with the electron affinity (EA).

The problem is Heats of Formation versus Total Energies as described in http://openmopac.net/manual/HoF%20versus%20total%20energies.html .

Is there a necessity to convert the heat of formation to the total energy first ?

$\endgroup$
2
  • 2
    $\begingroup$ It really depends on hat you are doing, how large the systems are and how accurate you want it to be. It is one approach and it'll probably is a well enough approximation for many things. In that sense Koopmans' Theorem comes to mind. While I don't think the question is off-topic here, it might do better on Matter Modeling. Just raise a flag if you want the question migrated ( and pleas do not cross-post it). $\endgroup$ Jan 7 at 21:59
  • $\begingroup$ If you use DFT those are just the HOMO and LUMO energies. $\endgroup$
    – Anon
    Jan 10 at 20:19

1 Answer 1

2
$\begingroup$

The simple answer is that the heat of formation difference between two charge states is the most conceptually and practically accurate way to estimate an ionization potential (IP) or electron affinity (EA) in MOPAC.

For a vertical excitation, you'd use the same (pre-excitation) geometries for both calculations (i.e. the 1SCF keyword), and for an adiabatic excitation, you'd relax the structure in each charge state. MOPAC will also report an IP and EA for each calculation, which is consistent with Koopmans' theorem for the semiempirical model Hamiltonian. Relative to a vertical excitation calculation, these quantities are missing orbital relaxation effects between charge states.

For the more advanced MOPAC user, the MECI functionality of MOPAC can be used to study electronic excitations with multi-reference character, whereas standard calculations are based on a single Slater determinant. However, these calculations are more complicated to specify, more computationally expensive, and may suffer from limited model transferability (the models in MOPAC are primarily fit to optimize the accuracy of single-reference calculations).

The heat of formation versus total energy discussion on openmopac.net is referring to the heat of formation reported by MOPAC versus the total energy reported by first-principles quantum chemistry software. While MOPAC can report a total energy quantity, it is not intended as an accurate approximation of first-principles total energies and is more of a historical artifact. I discuss this in more detail in a GitHub Issue. Recent versions of MOPAC hide the total energy output behind a keyword in an attempt to suppress this common source of confusion.

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .