convergence of SCF calculations for metallic systems in CP2K

S. Ling lsl.m... at googlemail.com
Tue Oct 4 12:00:33 CEST 2011


Dear Marcella,

Thank you very much for your information. They are very helpful.

Best wishes,

Sanliang


On 10月4日, 上午9时05分, marci <marc... at pci.uzh.ch> wrote:
> Dear Sanliang,
>
> Yes, you can run simulations with metallic systems and yes normally
> metallic systems are harder to converge.
> Since k-point are not implemented with CP2K, meaningful results can
> be
> obtained only with large enough supercells.
> For a metallic system, you have to consider the advantages of
> smearing
> the occupation number of the states around the Fermi energy.
> We use a Fermi Dirac smearing where the temperature parameter sort
> of
> determine how many states are involved in the smearing, and can
> affect
> the convergence.
> In order to do that the only possibility at the moment is to compute
> at each step of the SCF minimization eigenstates and eigenvalues of
> a
> number of KS orbitals larger than the number of electrons, in order
> to
> include some unoccupied states as well. Haw many additional states
> you
> need depends on the system and on the smearing temperature.
> This means that efficient algorithms as OT cannot be used, and you
> have to resort to standard diagonalization, which scales much worse
> with both system size and number of tasks.
> Another useful technique is the mixing of the electronic density in
> reciprocal space, to mend effects related to the charge sloshing.
> Moreover, to get well converged states you might need to tighten the
> convergence criterion (EPS_SCF).
> All this can make the calculation quite lengthy.
>
> A typical &SCF input section for a calculation with metals reads like:
> &SCF
>      SCF_GUESS  RESTART
>      EPS_SCF 8.0E-7
>      MAX_SCF 500
>      ADDED_MOS  700
>      CHOLESKY INVERSE
>      &SMEAR  ON
>          METHOD FERMI_DIRAC
>          ELECTRONIC_TEMPERATURE [K] 300
>      &END SMEAR
>      &DIAGONALIZATION
>           ALGORITHM STANDARD
>      &END DIAGONALIZATION
>      &MIXING
>           METHOD BROYDEN_MIXING
>           ALPHA   0.1
>           BETA    1.5
>           NBROYDEN  8
>      &END
> &END SCF
>
> best
> Marcella
>
> On Oct 3, 5:28 pm, "S. Ling" <lsl.m... at googlemail.com> wrote:
>
>
>
> > Hello all,
>
> > I am preparing to start some new calculations for a very large system
> > in which we have silver as substrate. I heard some comments saying
> > that the SCF calculations converge very slowly for metallic systems in
> > CP2K. I want to check if any of you have such experience with CP2K. If
> > yes, could you tell me how did you deal with this problem? Any hints
> > on special keywords to be set in the inputs? I also heard about some
> > new developments of CP2K to address this problem. Could you give me
> > some update suggestions?
>
> > Thank you very much.
>
> > Sanliang- 隐藏被引用文字 -
>
> - 显示引用的文字 -


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