[Wien] constrain some atoms to minimize the forces

[Stefaan Cottenier]

> Your question is based upon a common misunderstanding of minimizing
> forces. You are thinking that the time to optimize depends upon the
> number of positions (3*Number of Pt atoms that move) which is wrong.
> With PORT it scales as the number of clusters of phonon frequencies and
> weakly with the width of the clusters; with MSR1a it scales as the
> number of clusters of eigenvalues of the joint electron-phonon problem
> and their width.
>
> You will gain far more by generating a proper symmetric model with P-3m1
> (maybe P-31m) symmetry rather than what you appear to have, a P1
> structure. This will reduce the number of uniques phonon etc modes by
> something like 6-12, and will be much faster and converge better.

Dear Laurence Marks,

When having created the most symmetric slab model that is relevant for 
the case at hand, wouldn't you agree that it still makes sense to fix 
the positions of the middle few layers to their bulk positions? One 
cannot make slabs as thick as to find spontaneously bulk behaviour in 
the center, and therefore imposing the bulk geometry in the slab center 
could be preferred over getting unphysical lattice spacings. Can you 
comment?

Thanks,
Stefaan


-- 
Stefaan Cottenier
Center for Molecular Modeling (CMM) &
Department of Materials Science and Engineering (DMSE)
Ghent University
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