[Wien] MINI

[Stefaan Cottenier]

> I'm trying to generate a case.scf that will have forces in order to use
> mini positions.  I've inserted an impurity and still no induced forces,
> i guess this is because i'm using a "fixed" space group with no free
> structure parameters, unlike TiO2.
>
> How do i generate forces from an impurity/vacancy in a typical space
> group like zincblende or diamond?  Am i limited to only those space
> groups with free structure parameters?

You probably inserted the impurity in a cell with the same size as the
original cell? Or in a cell that is just slightly larger? For instance, if
you have only two A-atoms in your cell, and you replace one of them by an
impurity B, then chances are high that this will result in a highly
symmetric AB-structure with no free parameters. However, that is not whay
you want for an *impurity*, as your AB-structure will contain 50% of
'impurities'... This is an alloy instead.

What you have to do is to make a supercell first (say 2x2x2, then you would
have e.g. 8*2=16 A-atoms), and only then replace one of the A by B. For
sure, you will have forces then.

> How then would you effectively relax an impurity within a typical space
> group structure, will volume optimization suffice?

If you don't mind you have an alloy, then volume optimization is OK.

Stefaan