VCA setup

[musanna_galib]

Hello,

I’m trying to run a relaxation using the Virtual Crystal Approximation (VCA) in VASP 6.3.0 for a mixed-anion system with 5 elements, but I’m a bit confused about the correct setup. Here's what I have so far:

Setup:
In the INCAR, I added:

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VCA = 1.0 0.79 0.07 0.07 0.07

In the POSCAR, I started from a LiS₂ structure and wanted to replace the S atoms with a mixture of 4 atoms. Here I just changed S to S_vca:

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Li   S_vca  
8    4

In the POTCAR, I concatenated pseudopotentials for all five elements, assuming that the weights in the VCA tag would take care of the mixing.

However, when I check the OUTCAR, it seems that only Li and S (with weights 1.0 and 0.79) are actually considered. I don’t see anything regarding other elements

My Questions:
Do I need to copy and paste the S atomic coordinates 3 more times in the POSCAR to account for other 3 elements?

I’m planning to use VCA for a migration energy barrier calculation using NEB. Does using VCA make sense in that context?

Is it meaningful to compute DOS or charge density difference with a VCA setup, given that VCA may not accurately capture electronic mixing effects?

I’ve attached the OUTCAR for reference. Any clarification would be appreciated!

Thank you.

[andreas.singraber]

Hello!

Please always provide a full set of all relevant input and output files (see forum posting guidelines). Otherwise it is really difficult for us to find issues with the INCAR tags, your workflow or VASP itself. So far I can only see from the OUTCAR file that VASP does only recognize two distinct elements in your POSCAR (see here how the ion species and numbers should be provided):

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ions per type =               8   4

For those two types the first two entries in your POTCAR file are used, which are Li and S. Hence, it seems that the remaining weights in the VCA tag are probably unused.

All the best,
Andreas Singraber

[musanna_galib]

Hello Andreas,

I added all the setup files here. Can you please take a look at the issues?

My Questions:
Do I need to copy and paste the S atomic coordinates 3 more times in the POSCAR to account for the other 3 elements?

I’m planning to use VCA for a migration energy barrier calculation using NEB. Does using VCA make sense in that context?

Is it meaningful to compute DOS or charge density difference with a VCA setup, given that VCA may not accurately capture electronic mixing effects?

Thank you.