[Pw_forum] A question about relaxing interfaces

[peressi at ts.infn.it]

Due to periodic boundary conditions, you have 2 interfaces
(repeat your cell along the growth direction and you will see immediately),
that may be equivalent or not according to their details (crystalline
structure, crystallographic direction, composition of the interface
itself...).

Coming to your question:
If the in-plane lattice constant is fixed, you should optimize c
(='whole z lattice constant'). A good starting point is determined by
calculating
the z lattice constant separately of bulk1 and bulk2 constrained on that
in-plane
lattice constant , using the macroscopic theory of elasticity and using
the average
at the interface region.
Then, you calculate the corresponding stress and if the z-component is non
negligible
you will slightly modify c (rescaling the internal atomic position; check
forces on the atoms;
if they increase too much, you will need also to re-relax them) and
recalculate the stress.
The optimal c could be found by linear interpolation.
You could look at a review article and references therein:
M. Peressi, N. Binggeli, and A. Baldereschi, Band engineering at interfaces:
theory and numerical experiments. Phys. D: Appl. Phys. 31, 1273 (1998).

Maria


>
> Dear all PWSCF users,
>
> I am trying to do some relaxation calc. of interfaces with PWSCF, but
> these's a question wrt relaxations:
>
> Suppose we have a supercell without vacuum layers and only one
> interface (i.f.), which might look like this:
>
>         -------
>         |     |
>         |bulk1|
>         |     |
>         | i.f.|
>         |     |
>         |bulk2|
>         |     |
>         ------
> There are strong inplane constraints, so we only relax distances between
> atomic layers at the interface. So here comes my problem: if we use
> "relax", the atomic layers at the interface will be relaxed, but since the
> whole z lattice constant won't change, the structure is supposed to be not
> fully relaxed. Is this right?
>
> Otherwise, if we use "vcrelax", how could one set up inplane constraints?
>
> Thanks a lot.
>
> Bests
>
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://www.democritos.it/mailman/listinfo/pw_forum
>