convergence of SCF calculations for metallic systems in CP2K

[S. Ling]

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

[marci]

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

[S. Ling]

Dear Marcella,

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

Best wishes,

Sanliang

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