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I want to perform redox potential calculations using the Gaussian. To check if I'm setting things up correctly, I want to compare it to a benchmark database published by Grimme et al.[1] I performed all calculations with my functional/basis set combination for the gas phase molecule.

Now I need to perform the calculations for the species with charge $+1$. I choose molecule number 45 from the OROP Set (Supplemental Information), it is Hydroquinone. The paper said it should have multiplicity 1 and charge +1. But I think I have to remove an atom to get the combination of M=1 and Ch=+1.

Can somebody help me out, which atom do I have to remove? Thanks in advance!

I also looked up the zip-dataset, which should include (according to the README) the geometries (these can I see), but also the charge and multiplicity data, which I can not find in the zip file! Are they missing or can my Mac not show them?

References:

  1. Neugebauer, H.; Bohle, F.; Bursch, M.; Hansen, A.; Grimme, S. Benchmark Study of Electrochemical Redox Potentials Calculated with Semiempirical and DFT Methods. J. Phys. Chem. A 2020, 124 (35), 7166–7176. DOI: 10.1021/acs.jpca.0c05052.

Here are my calculated Energies. (II) is the normal Molecule, (III) is with charge +1:

SCFE (g) (II) =     -382.81331295
SCFE (solv) (II) =  -382.82970187
GibbsCorr (II) =    0.077174
SCFE (g) (III) =     -382.53186782
SCFE (solv) (III) = -382.63171676
GibbsCorr (III) =   0.078470
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In the dataset files for a given molecule, the charge and number of unpaired electrons for each species are stored in files .CHG1/.CHG2 and .UHF1/.UHF2 respectively. On some computers, files starting with . are hidden by default. On Mac, you can display these in Finder by pressing <COMMAND><SHIFT>..

Looking at these for molecule 45 of the OROP set, it says that the charged species should have a $+1$ charge and $1$ unpaired electron (i.e. a multiplicity of $2=2\cdot\frac{1}{2}+1$). So you shouldn't need to remove any atoms to make this a valid charge/multiplicity, but you will need to perform an unrestricted calculation. You shouldn't need to do anything special for that, as Gaussian will run an unrestricted calculation whenever the multiplicity is greater than 1. From the paper, they didn't seem to run unrestricted calculations for the closed shell molecules (and in the vast majority of cases it would just give the same result as a restricted calculation any way).

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  • $\begingroup$ Thanks for the comment. So I looked up the hidden files. I tried different combinations in gaussian without deleting a atom of my molecule. I only can choose a doublet which is given with multiplicity 2 in gaussian. I will edit my post to ad the jobfile and an excel file where I tried to calculated with this calculation the redox potential. The experimental reference for molecule 45 is 1.1V. But I calculated always values around 5V. Far away... $\endgroup$
    – Andrea
    Jan 12 at 21:28
  • $\begingroup$ Please don't mind that there is no 40 in the jobfile. In first try I made a mistake with the numbering of the molecules in the supplemental material. It is the right molecule. $\endgroup$
    – Andrea
    Jan 12 at 21:36
  • $\begingroup$ @Andrea I had a typo in the answer, multiplicity 2 should be correct. $\endgroup$
    – Tyberius
    Jan 12 at 21:39
  • $\begingroup$ @Andrea As for the issue of your calculations not producing the correct redox potential, that should probably be put as a separate question since at least the initial question "how to modify the charged species" is solved here. $\endgroup$
    – Tyberius
    Jan 12 at 21:45

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