general question on calculating metals using cp2k

Jun chen... at gmail.com
Thu Nov 3 12:04:37 CET 2011


Thanks very much again for the prompt reply, Carlo.

I agree a thickness of 4 layers is a minimum. I hope the screening of
metals is strong enough to eliminate the interaction between two
surfaces. I know termination with H atoms to saturate surface dangling
bonds is often used for covalent solids with a band gap. Is this
necessary for metals?

I will definitely do some tests on the size of slab before I really
run the calculations.

Cheers,
Jun

On Nov 3, 10:43 am, carlo antonio pignedoli <c.pig... at gmail.com>
wrote:
> Dear Jun form the experience I have for systems I worked on,
> a slab 4 layer thick could be the minimum thickness for a first investigation,
> be careful that you would have, in such a thin slab, a spurious interaction
> among surface states.
> To reduce this, a "empirical" way is hydrogenation of one side of the slab.
>
> Dont' forget to consider the relevance of van der Waals corrections.
>
> Usually the smallest slab I consider in xy is in the order of 6x6
> compared to a primitve cell
> but this is just an indication and the result depend really on the
> particular system you are dealing with.
>
> Magnetic systems are always complex.. I have not enough experience on
> this to give
> you suggestions.
>
> Ciao
>
> Carlo
>
>
>
>
>
>
>
> On Thu, Nov 3, 2011 at 11:34 AM, Jun <chen... at gmail.com> wrote:
> > Thanks, Carlo.
>
> > Yes, I want to do slab calculations. The properties I am interested in
> > are, for example, adsorption energies of some atoms or molecules, work
> > function, DOS et al. For start, I guess the thickness of four layers
> > should be enough, I mean for Pt(111). What I am not so sure is the xy
> > dimensions. Would MxM, say M=5, be enough? I read some discussions on
> > the magnetic state of Ni that Ni slab of M=6 and 6 layers is necessary
> > to get a reasonable spin state. I hope it is less demanding for non-
> > magnetic metals like Pt.
>
> > Cheers,
> > Jun
>
> > On Nov 3, 8:53 am, carlo antonio pignedoli <c.pig... at gmail.com>
> > wrote:
> >> Dear Jun,
> >> regarding the size of the cell, it depends on which property of the
> >> system you would like to reproduce.
> >> There is a simple rule:
> >> if your property is well converged with a k-point cacluation for a
> >> unit cell on a MxM regular grid not shifted (I mean
> >> including Gamma point..)
> >> then a supercell as big as MxM the unit cell will give you "exactly"
> >> the same results.
>
> >> Of course overriding symmetries could bring the MxM system to a
> >> geomoetry not compatible
> >> with the unit cell (this does not mean that the result is wrong...)
> >> that's in part why I was saying "exactly".
>
> >> If you are interested in slab calculations, no k-points in the
> >> direction perpendicular to the slab surface,
> >> then the thickness of the slab, again, depends "solely" on the
> >> particular properties of the system that you
> >> would like to reproduce.
>
> >> For the behavior of the optimization algorithms i let somebody more
> >> expert to reply.
>
> >> Ciao
>
> >> Carlo
>
> >> On Wed, Nov 2, 2011 at 6:35 PM, Jun <chen... at gmail.com> wrote:
> >> > Hi,
>
> >> > I intend to run some calculations on metals. I am wondering if experts
> >> > could give me some ideas on the performance of cp2k. For example, to
> >> > model a metal surface, say Pt(111), what size of the system is big
> >> > enough for compensating the lack of k-point sampling? Usually, how
> >> > many iterations are needed to reach SCF convergence? How long it
> >> > roughly takes for each SCF step? How slow is the matrix
> >> > diagonalisation compared to OT? ...
>
> >> > It would be nice if someone shows me an input example for metals. I
> >> > know there was one given by Prof. Hutter about two years ago. Any up-
> >> > to-date one?
>
> >> > Thanks a lot,
>
> >> > Jun
>
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