# Calculated Redoxpotential is too far away from experimental value

I calculated the redox potential with Gaussian for hydroquinone with different combinations of functional/basis sets always values like 5.4V (B3LYP/6-311G+(2d,p) and SMD model). The experimental value is about 1.1V according to , so I'm missing by a decent amount.

Has somebody an idea what I did wrong?

These are the energies I calculated:

SCFE (g) (molecule) =   -382.81331295
SCFE (solv) (molecule) =  -382.82970187
GibbsCorr (molecule) =    0.077174
SCFE (g) (species) =     -382.53186782
SCFE (solv) (species) = -382.63171676
GibbsCorr (species) =   0.078470


and an example job file for the optimization of the charged species:

%rwf=bench_40_spec_b3lyp_tight.rwf
%NoSave
%chk=bench_40_spec_b3lyp_tight.chk
#p opt=(calcfc,tight,recalcfc=3) b3lyp/6-311+g(2d,p) nosymm scf=qc

040 Tight Opt Gas

1 2
C                 -0.68613672   -1.20444633   -0.00000200
C                  0.69506215   -1.19541581   -0.00000235
C                  1.39161904    0.00718044    0.00000031
C                  0.68613843    1.20444943   -0.00000167
C                 -0.69505913    1.19541474   -0.00000053
C                 -1.39161802   -0.00718091    0.00000159
O                 -2.75490670   -0.05822072   -0.00000096
H                 -3.11159203    0.83776787    0.00004645
H                 -1.23719750    2.13172433   -0.00000924
H                  1.23422725    2.13274296   -0.00001200
O                  2.75490286    0.05821954    0.00000081
H                  3.11158561   -0.83777105    0.00003365
H                  1.23719782   -2.13172346   -0.00000066
H                 -1.23422866   -2.13274050   -0.00000041


I used the method linked in here: Redox Method

### 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.
• It would be helpful to know what you specifically you got for the potential using the energies in the question. One thing to note is the linked paper subtracts the potential for a reference electrode to obtain its results. I can't find in the paper what value that should have, but it may make up for most of the discrepancy. Wikipedia has an estimate of 4.44V for the absolute potential of the standard hydrogen electrode.
– Tyberius
Jan 12, 2022 at 22:19
• I got 5.4V, edited in the post. Jan 12, 2022 at 22:32

Your reference paper says that it subtracts the "absolute potential of the reference electrode" from the calculated values. I can't get to the citation that explains what reference electrode they are referring to, but the most common is the Standard hydrogen electrode, which is estimated to have an absolute potential of $$\pu{4.44V}$$.
Subtracting this from the potential of $$\pu{5.4V}$$ that you calculated gives $$\pu{0.96V}$$. This is only off by ~$$\pu{0.1V}$$, which is well in line with the errors reported in the paper for DFT and semiempirical methods.