POTCAR for H and He

[gu301purdue]

Dear there,

We would like to introduce H defects in SiO2 (quartz structure) and I realize there are many different POTCARs under potpaw_PBE.54, for example, H, H1.25, H1.33, H_AE, etc. What's the difference between them and which one I should use?
Besides, we also want to introduce 3He in the structure (not 4He). Do we have POTCAR for 3He?

Thanks a lot!
Tina

[marie-therese.huebsch]

Hi Tina,

On the following VASP Wiki the available PAW potentials are described and there is a specific paragraph about hydrogen:

Hydrogen-like potentials are supplied for a valency between 0.25 and 1.75, as listed in the table below. Further potentials might become available, and the list is not always up to date. Mind that the POTCAR files restrict the number of digits for the valency (typically 2, at most 3 digits). That is, using three H.33 potentials does not yield 0.99 electrons and not 1.00 electron. This can cause hole- or electron-like states that are undesirable. The solution is to slightly adjust the NELECT tag in the INCAR file.

So it basically depends on the valency you would like to use. Does this help?

Regarding 3He, you can use the POTCAR file of 4He and change the value of POMASS to 3.0. That is because, the electronic configuration is the same and only the ionic mass is different. Please, carefully consider if the effect you are trying to investigate is captured in the under theory.

Best regards,
Marie-Therese

[aohara]

In my experience, these fractional hydrogen pseudopotential are useful for termination of dangling bonds in slabs to passivate as if they were still connected to bulk. For example, if I had a GaAs slab, and I only wanted to study some physical effect on one surface of the slab, I could terminate the other slab with fractional pseudohydrogen. In the GaAs case, we would consider that the nominal valence of Ga is 3 and As is 5 and in GaAs both have 4 bonds (which requires 2 electrons). So using those consideration, one can consider that for each bond, As contributes 5/4 of an electron to the bond and each Ga contributes 3/4 of an electron to the bond. Therefore the dangling bonds at the surface can be passivated for As using the H.75 pseudopotential and for Ga the H1.25 pseudopotential. As a calculational example from my own work, I did this procedure for CdS slabs because I was interested in molecular absorption on just one of the surfaces and I could only make the CdS slab so thick (and not as thick as the experiment).

For defects based on hydrogen in a bulk material or if you are studying how hydrogen would really passivate a surface, then the regular H with 1 electron should generally be used as this is the more physically "real" version of hydrogen. So for example, if I have an oxygen vacancy in SiO2 or a nitrogen impurity and I wanted to see how hydrogen would interact I would use the "H" pseudopotential. Even in GaAs if I had a Ga vacancy and wanted to study hydrogen interactions I would use the regular hydrogen pseudopotential. As a calculational example, we recently studied hydrogen passivation of substitutional O for As defects in GaAs and in the past passivation of oxygen vacancies in oxides like HfO2. I know that other authors have looked at interstitial hydrogen and water in SiO2 as well as hydrogen vibrational signatures.

I hope this helps,
Andrew