[XtalOpt-devel] shell model in GULP

[David Lonie]

Hi Wei,

On Tue, May 15, 2012 at 10:01 PM, Wei Lai <laiwei at msu.edu> wrote:
> 1) It seems "gulpFracShell" forces both Mg and O to adopt core/shell
> models. How can we use core for Mg and core/shell for O?

This is not supported at this time -- the keyword replacement will
either print core/shell for all atoms or none. I've not used
core/shell calculations much, but there may be a way to "trick" GULP
into neglecting the Mg shells.

> 2) If we already know the positions of Mg and we just want to relax O
> positions, how can we apply fixed positions (fixed fractional
> coordinate) to Mg only?

I've typically used XtalOpt on completely unknown systems, and as a
result never implemented a way to fix the coordinates. I'm not really
sure how well this work with an evolutionary/genetic algorithm either,
as I've only tested the operators used to generate new structures in
cases where all atoms are free to move. If you have a mostly solved
structure with just a few unknown atoms, I think the best way to use
XtalOpt would be to generate several guesses at the structure using
the information that you know, and then use these to seed the first
generation of the search. Otherwise, it may be best to come up with
some way to randomly generate and optimize trial structures that
explore only the unknown structural parameters.

Probably not the answers you were looking for, but I hope this helps!

Dave