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I am using Quantum Espresso to simulate a platinum surface with ligands, so far everything is working ok. I now need to introduce a solvated hydronium $\ce{H_3O^+(H_2O)_2}$.

Is there something I should be careful about when introducing charged species? I will add the keyword tot_charge = +1 but I see that I could also use starting_charge(i), i=1,ntyp. Which one is preferred?

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1 Answer 1

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Personally, I prefer to use the second option.

The full description of first option is:

tot_charge  REAL
Default:    0.0

    Total charge of the system. Useful for simulations with charged cells.
    By default the unit cell is assumed to be neutral (tot_charge=0).
    tot_charge=+1 means one electron missing from the system,
    tot_charge=-1 means one additional electron, and so on.

    In a periodic calculation a compensating jellium background is
    inserted to remove divergences if the cell is not neutral.
             

So, with this option, you are indicating that your cell is not neutral but without specifying where the charge is located.

On the other hand, the full description of the second option is:

starting_charge(i), i=1,ntyp    REAL
Default:    0.0

    starting charge on atomic type 'i',
    to create starting potential with startingpot = 'atomic'.
             

In this case, you are in control where the charge is initially located. Also, you can specify the potential (keyword startingpot) being calculated from the charges of the system or from a file (previous calculation):

startingpot     CHARACTER

     Available options are:
                

    'atomic' :

        starting potential from atomic charge superposition
        (default for scf, *relax, *md)
                    

    'file' :

        start from existing "charge-density.xml" file in the
        directory specified by variables prefix and outdir
        For nscf and bands calculation this is the default
        and the only sensible possibility.
                    
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