# How to alter the charge and multiplicity of a molecule in Gaussian?

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


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).