[DFTB-Plus-User] DFTB-MD with surfaces and amorphous materials

[Bálint Aradi]

Dear Vanessa,

>
> (1) Can DFTB+ be used to perform MD simulations with surfaces? I gather
> from one of the example MD simulations, 'SiH-surface', that comes with the
> latest version (v 1.2.2) that DFTB+ is capable of performing simulations
> with surfaces. However, when I tried a simple test, in which I took the
> input from a well-behaved DFTB-MD simulation and increased the lattice
> vector along the z-axis (to reflect a surface), I found that the first time
> step of the simulation doesn't converge. (See input below. I'm using
> version 1.2.2.) Am I correct about my assumption that DFTB+ can perform
> DFTB-MD simulations with surfaces? If so, what might I be doing incorrectly?

Yes it can. One of the possible problem with your approach is, that you
just "tear" your structure apart (by increasing the z-lattice vector),
so that you end up with a lot of dangling bonds. In that case, setting
the electronic temperature to 1 K is not enough, you are warrantied to
get serious SCC/SCF convergense problems with any electronic structure
method. Settig a higher electronic temperature (e.g. 300 K or above)
could solve the issue.

> 
> (2) My other somewhat related question is about whether or not DFTB+ is
> appropriate to perform DFTB-MD simulations with amorphous systems. I assume
> that this is true if the system is treated as a non-periodic (or "cluster")
> system, but is it possible to treat them as a periodic system?

Sure you can. Again, DFTB is nothing special in that regard, you can do
it with it as with any other code having periodic boundary conditions.

  Best regards,

  Bálint


-- 
Dr. Bálint Aradi
Bremen Center for Computational Materials Science, University of Bremen
http://www.bccms.uni-bremen.de/cms/people/b_aradi/


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