Least popular vibrational entropy model provides the best accuracy and robustness

What is it about?

Our article is about vibrational free energies, which are vital for making reliable computational predictions in all fields of chemistry. We show that, although low frequencies cannot be computed accurately using current quantum chemical approaches, this problem can be solved by trading physical background for robustness.

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Photo by MARIOLA GROBELSKA on Unsplash

Photo by MARIOLA GROBELSKA on Unsplash

Why is it important?

Accurate prediction of chemical reaction pathways using quantum chemistry requires proper evaluation of vibrational contributions to the free energy. It is two key points in this context: 1) The classical harmonic approximation, commonly used in Gaussian and most quantum chemistry programs, is highly sensitive to low-frequency vibrational modes. A shift of just 35 cm⁻¹ in these modes can alter the free energy by more than 1 kcal/mol. 2) These low-lying frequencies usually depend on solute’s interactions with the solvent and so are fundamentally unreliable for any chemical system in solution computed with implicit solvation (as done in most cases). This issue persists for many quantum chemical methods, even for most accurate ones. However, quasi-harmonic Truhlar’s approximation, although being unpopular, can work under this uncertainty both reliably and accurately. Moreover, our minor modification to this model further improves its performance.

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