Using VCA method for 2 different occupancy ratios

[mtardieux]

Dear VASP users,

I have a question concerning the Virtual Crystal Approximation (VCA) method.

In the explanation provided on the Wiki page, VCA is demonstrated with a two-atom cell to simulate Sn doping in a Ge lattice. I was wondering how this approach would work in cases where we want to mix two sites with different occupancy ratios (to introduce disorder on two different sites with different occupancies).

Let me describe my specific scenario:

I have a POSCAR file consisting of three atomic species: A, B, and C. Species A has an occupancy of 1, so if I understand correctly, the VCA parameter would be set as: VCA = 1.0 …. ….

However, for the sites occupied by B, I would like a composition of 65% B and 35% C.
On the sites initially occupied by C, I would like to adjust the composition to 15% B and 85% C.

Does anyone know how to set this within VCA?

Best,

Marie

[jonathan_lahnsteiner2]

Dear Marie,

I think the tag INCAR you are looking for is VCAIMAGES. In this way you will have to prepare two simulation folders called 01 and 02. The folder 01 should contain the pure AB and the simulation folder 02 should contain the pure AC composition. Now you can set VCAIMAGES to some real value and get the partial occupancies you are aiming for. For more information please check the wiki page.

All the best Jonathan

[mtardieux]

Dear Jonathan,

Thank you very much for your answer.
I had a look at the VCAIMAGES parameter you sent me, and tried to think about its use for my problem.
However, if I understood the information correctly, it seems that this parameter would allow me to specify only one of the two fractional occupancies, enabling me to describe just one of my sites (for instance B sites being occupied by 65% of B atoms and 35% of C atoms). Unfortunately, it would lead the second site, with its different occupancy, unaccounted for.

Am I interpreting the information correctly?

Best,
Marie

[jonathan_lahnsteiner2]

Dear mtardieux,

With the VCAIMAGES tag you can set up two simulations. Let's call them for example S1 and S2. Those will be stored in folders 01 and 02. By setting the tag VCAIMAGES to, x
you will obtain a simulation S which is averaged over the two folders as S = xS1+(1-x)S2. If this allows you to realize the occupations you are describing in your post will depend on the used POSCAR file. In principle it should be possible two prepare to POSCAR files where their mixture gives the occupancies described by you. Additionally it might be useful for you to look into the INCAR tag IMAGES. Here you can prepare an arbitrary number of different occupancies in your POSCAR files by hand.

All the best Jonathan