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wham09

Member

Registered: 2025-09-19

Posts: 57

Applying Marcus theory by QM calculation

Dear Prof. Lu,

I would like to apply semi-classical Marcus model to evaluate solution-state electron transfer rate constants, for which I have multiple questions.

1. Let's say the ET reaction is A + B -> A(+) + B(-). In principle, should I calculate two sets of driving forces (delta_G), Huang-Rhys factors (S), the corresponding frequencies (omega), and reorganization energies (lambda) from separate calculations of A, A(+), B, B(-), and then combine them somehow?
Or should I optimize the geometry of A...B bimolecular complex, and then optimize A(+)...B(-) complex by TD optimization (following the CT state) and finally calculate one set of the factors?

2. In your blog post (sobereva.com/330), you stated that lambda_outer is difficult to calculate and often approximated. But in another post (sobereva.com/333), you presented quite clearly how to calculate it. Is the method presented in the article 333 indeed the standard way? Or is the accuracy of this approach questionable?

3. From your lecture notes and blog posts, I think I have some understanding of how to calculate all the terms required in the semi-classical Marcus equation, except the electronic coupling (NAC or H_ab). Is it correct that the most rigorous way to get this is to optimize the MECP geometry of the A…B complex, at which the intermolecular electron transfer actually happens, and then obtain the eigenvalue difference?
If there is other way that is more standard or commonly used, can it be done using Gaussian or ORCA?

4. If I wanted to calculate rate of outer-sphere electron transfer between an organic reactant and an electrode (in a solution of electrolyte in organic solvent), I could imagine that there will be many more uncertainties, such as:
  1) how to optimize the distance between the electrode and the reactant at the moment of ET, and how to simulate the double layer environment at this distance
  2) how to optimize geometry of the reactant in the presence of electric field, organic solvent, and electrolyte
  3) how to calculate the terms in the Marcus equation when electrode is involved
About this question, I'm basically a novice. Can all these be addressed by QM methods (desirably using Gaussian or ORCA)? If not, is there a less rigorous, but good enough QM routine that researchers use?

Last edited by wham09 (2026-03-04 14:51:01)