Implicit self-consistent electrolyte model

[JGRiver]

As starting point, and since the iterative calculations with the electrolyte are computationally demanding, I decided to perform some test with a simple system by selecting a molecule (OCHO), before implementing on the actual systems. In this case, after performing a structural optimization, I include the solvent with the following parameters: NC_K = 0.0025, SIGMA_K = 0.6, TAU = 0.0 and EB_K = 78.4, obtaining a Fermi energy Ef=-6.9731eV.

The next step was modifying the total number of valence electrons, (in this case NELEC=17) by increasing the value of NELECT by 0.01 until NELECT=17.1 and decreasing by 0.01 until a final value of NELECT=16.9.

In this case, I would like to ask how to associate and calculate a potential associated to the shift of the Fermi level for each case for example:

for NELEC=17 the resulting fermi energy amounts to Ef=-6.9731 eV, then for NELECT= 17.01 the fermi energy amounts to Ef= -6.9598 eV , hence the shift in the energy level ( -6.9598 - (-6.9731)) corresponds to 1.33E-02 eV. Here, if I use the standard definition of Volt, I will need to divide the 1.33e-2 eV by the elementary charge of an electron (1.6e-19 V) is this correct?, I assume that I am missing something since the value does not have an adequate magnitude.

Thanks in advance,

Jose Guillermo

[rhennig]

If you change the molecule's charge by a fractional amount, you will start filling the LUMO state when you add electrons and empty the HOMO state when you remove electrons. For molecules, you should also think about the change in spin state when adding or subtracting electrons. As a consequence of the HOMO-LUMO gap, you should see a jump in the Fermi level in your calculations when you add electrons. Your results do not seem to show this, which may be a consequence of how VASP calculates the Fermi level. What are you using for ISMEAR?

Finally, since we express energies in eV and potentials in V, the charge of a single electron is 1, not 1.6E-19. This choice for the unit of the charge makes comparisons between energies and potential seamless.

[JGRiver]

Dear rhennig, Thanks for your answer. I have solved that issue, and indeed as you pointed out I had some problems by defining e (the formal charge of an electron which for general terms will be equal to 1 since the Volts are defined as eV*e-1 where e-1 is the elementary charge. So going back to my example:

Bias voltage (or electrode potential) = - (efermi + FERMI_SHIFT + 4.6) ; units V is equal to the desired potential in Volts (4.6 for PBE functional)

for instance in my example the NELECT= 17.01 the fermi energy amounts to Ef= -6.9598 eV with a FERMI_SHIFT = 4.737053684306013E-003 and the total electronic energy E= -23.50472eV

the bias potential will be equal to =-(-6.9598+4.7370e-3+4.6)=2.35V with an energy of =-23.50472 +(-4.73e-3*0.01)= -23.50472 eV

I assume that for larger modifications on NELECT the correction to the energy will become more relevant.

On the other hand, my actual interest is to model reactions like H+ +e- CO2+ --> COOH ( where H+ is a proton, e- an electron and * a surface site) however in this case, I dont have clear how to tackle this systems, for instance: i) should I study each part individually, H, CO2 and slab (at the desired bias voltage), ii) should I study all the species together in the same system (slab+H+CO2). Regards Jose