What is the artifact of the Pt valence density referred to in example04 of the Environ module?

Question

I am trying to model a metal oxide system with an adsorbate in Quantum Espresso with the Environ module. For this, I am following an example from the module itself, example04 which

shows how to use pw.x to model 2D periodic systems in contact with a continuum solvent. (from README file)

The file further goes on to state that:

Note that the keyword solvent_mode='full' is mandatory in this calculation, in order to remove the artifact of the Pt valence density, which presents a hole close to the position of ion, due to the missing core electrons. In order to avoid the continuum solvent to enter such a hole, a quickly vanishing gaussian density is added at the position of the nuclei when defining the dielectric. The same problem is likely to occur in halogens and other transition metals.

Can anyone give some idea as to what this artifact is in the Pt valence density, the hole close to the ion, and missing core electrons are?

(The pseudopotentials used in the example are: O.pbe-rrkjus.UPF C.pbe-rrkjus.UPF Pt.pbe-nd-rrkjus.UPF)

Any insights would be greatly appreciated.

Answer 1

I saw your post on the google group. I guess you haven’t gotten a response there yet. So the problem stems from the fact that pseudopotentials don’t have core electrons. If you were to compute the electron density using pseudopotentials, you’d see spherical holes centered on the nuclei with radii equal to the radial cutoffs of the pseudopotentials.

Now why do we want to remove core electrons? Because it greatly improves the computational cost of the simulation, and since core electrons don’t participate in chemical bond formation, it’s safe to do so. If you left the core electrons (that is to say, you performed an all electron calculation) you’d need to use extremely high planewave cutoffs to capture the small / high frequency oscillations associated with core electron orbitals and the associated potential. This is standard practice in all planewave pseudopotential DFT codes.

Now what Environ tries to do is to fill the vacuum region in a DFT cell with an electron density-dependent dielectric permittivity function. The problem with this is that, with pseudopotentials, there is essentially vacuum in the core region due to the missing explicit electrons there. So to prevent implicit solvent from filling the core regions, auxiliary Gaussian charges are added to trick the code into thinking there is finite electron density in the core region, thereby preventing solvent from being accidentally added inside of the atoms.