VCA for elements with different valence electrons

[gherasi]

I understood from a previous post that VCA for elements with different valence electrons is not available.
What would you suggest it can be used to estimate the lattice changes for alloys of elements with different # of valence electrons?

[Petr Khomyakov]

If your alloy structure is random alloy, you may then use a supercell approach for alloy modeling, combined with special quasirandom structure (SQS) method to avoid doing multiple large-supercell calculations, see Section (and references therein) on QuantumATK application for SiGe alloy in our recently-published QuantumATK reference paper at https://iopscience.iop.org/article/10.1088/1361-648X/ab4007 - it is an open access paper, so you may download right away. Note that SQS methodology is implemented in QuantumATK. You just have to decide on your alloy supercell size, generate an SQS structure of that alloy, and proceed to regular geometry optimization (volume and/or ion relaxation) to get an equilibrium lattice parameters of an alloy for a given alloy composition.

We still have a plugin in the Builder that can create a supercell structure with randomly-placed alloy atoms, e.g., Ge in Si, but then you would need to generate several of those for a given alloy composition to have some statistics. Using SQS allows reducing the computational burden by a smart choice of an alloy structure, but one still have to consider a supercell size large enough to get an accurate description of physical quantities of interest. Volume would probably be not that sensitive to that. So, one may consider a supercell of a moderate size. But that may depend on your system of study.

[gherasi]

Is there a tutorial regarding the supercell approach for alloy modeling, combined with special quasirandom structure (SQS) method ?

[Petr Khomyakov]

Here is a manual page on SQS, https://docs.quantumatk.com/manual/Types/EvolutionarySQS/EvolutionarySQS.html, see also a reference therein. Otherwise, there is no particular difference between regular supercell calculations and supercell calculations for an SQS structure, as soon as the latter is generated as described in the manual and literature.

I note that there exists GUI support for SQS generation, see the corresponding plugin in the Builder.

[gherasi]

Questions:
1) What is the name of the plugin in the Builder that is used for SQS generation?
2) To what version of QuantumATK was the plugin added?
3) Is it possible to add the plugin to an earlier version?
4) Is there an example for how a hexagonal ternary alloy, A(x)B(1-x)C, lattice should be defined before the optimization with EvolutionarySQS ?

Thank you

[Petr Khomyakov]

It is in Builders -> SQS Alloy Builder. It is not possible to add it to older versions, as it is not an Add-On plugin. In fact, it requires the actual calculation using Job Manager, meaning that it is a very special plugin. So if you do not have it, the only way of adding it is to upgrade to a latest version of QuantumATK.

I have attached a png-file with an image how to set up SQS optimization in the Builder. Essentially, you have to create a supercell structure (any symmetry, cubic, hexagonal ..., or with no symmetry at all) of a pristine material in a usual manner, e.g., Si, then select a region which you want to alloy, e.g., all atoms to alloy the entire structure, and choose an alloy element, e.g., Ge, and set composition and other settings, if needed. Send the thing to Job Manager (or the Editor if you want to see the actual script). The SQS configuration will eventually appear as an object on the LabFloor.

[gherasi]

How do I get the details related to an upgrade?
It seems that currently the SQS alloy can be defined only for binaries. It this right?
What about ternaries?

[Petr Khomyakov]

- You may apply for a free trial first at https://www.synopsys.com/silicon/quantumatk.html. Which version do you have now?

- You can still do A_x B_1-x C, just select only the elements A and B for mixing in the SQS alloy plugin. It is only if you want to do A_x B_y C_1-x-y, i.e., mixing all the 3 elements at the same time, then there is a limitation of the current implementation.

[gherasi]

2016.3 Is the free trial fully functional?
How does one specifies the 3rd element, C?

[Petr Khomyakov]

- The latest version of QuantumATK is 2019.12. No other version is distributed. Version 2016.3 is not even supported any longer.

- To create A_xB_1-x C alloy, one should create AC supercell, select A elements, and choose B for Element 2 in the SQS alloy plugin, and do all the steps described in a previous post. 

[gherasi]

I have a perpetual license.
How would an upgrade relate to that?

[Petr Khomyakov]

If you have VNL-ATK 2016.3 (from QuantumWise that was acquired by Synopsys in 2017) with whatever license and want to have a recent QuantumATK 2019.12, you have to apply for a trial version using the link at the page I previously mentioned. Your application has to be approved by Synopsys, and after that you will be able to get an access to the trial version  through https://solvnet.synopsys.com/.

I note that having a perpetual license does not mean that you have a free update to newer versions. It is however free to update to the follow-up SP (service pack) version that includes bug fixes.