Hello,
In the QChem 6.3 manual (and earlier versions), under section 10.7.1.2 (“Interpreting the Output” for harmonic vibrational analysis), it is mentioned:
“For thermochemical calculations performed in implicit solvent (using models described in Section 11.2), there is a subtle point … whether vibrational contributions to the free energy should be included or not… There is a potential double-counting problem if TSvib from a vibrational frequency calculation in implicit solvent is added, which might be avoided by instead using gas-phase harmonic frequencies for the solute…Note that Q-CHEM’s harmonic frequency engine assumes a gas-phase molecule (even in the presence of continuum solvent), so that rotational and translational contributions to the free energy are printed out in every case. These should not be included in solution-phase free energy differences.”
I am trying to calculate ground and excited-state redox potentials, and want to make sure that all correction terms are accounted for correctly.
So does this mean that if I perform a geometry optimization and frequency analysis of a molecule within PCM, then the SCF energy (which includes the electronic energy and the free-energy of solvation) is the only term that includes solvent corrections? Because it seems to me that if the frequency engine considers a gas-phase system, then Hvib would correspond to the gas-phase, as would Svib. Also, if it is recommended not to use the rotational and translational terms for solution phase free-energy differences, how does one account for those contributions? Some clarity about these terms (that typically feature in a thermodynamic cycle for redox reactions) would be greatly appreciated.
Best,
Bushra
Hi Bushra,
The PCM (solvent) corrections are definitely included in Hvib, so that the Svib that you get is based on a harmonic partition function that uses solution-phase vibrational frequencies. The statement that you mention in the manual is simply trying to point out that translation and rotational entropy corrections may not be appropriate in a condensed-phase system, as they are based on particle-in-a-box or rigid rotor models. There’s some discussion of this point, with references to the relevant literature, in my PCM review:
https://doi.org/10.1002/wcms.1519
Hello John,
Thank you for that clarification. I took a look at the literature and if I understand correctly, the gas-phase vs. solution-phase contributions are debated about when using thermodynamic cycles, rather than the “direct method” of subjecting all species individually to geometry optimization and frequency analysis in a PCM.
So just to confirm, if I calculated the free energy of a species in solution using:
G= E(scf energy with pcm contribution) + “Total Enthalpy” + “Total Entropy”,
and then used differences of these values to compute values like redox potentials, that would be theoretically sound? Also, if I wanted to make it a bit more accurate, I could replace the “Total” enthalpy and entropy terms, with just the vibrational components? I want to really make sure I understand the correctness of including/dropping any terms.
Thank you so much, as always!
what you have described are the options. whether it is more accurate to drop the trans/rot part is debated. Including it seems theoretically questionable to me but last night I spoke to a student in the Grimme group who is working on solvation models, and in his hands he gets better results if these are included.
Alright, thank you so much for the clarification. I have been looking at some redox potentials, and the correction terms tend to increase MAEs with experiment, so I was curious.