[Pw_forum] ensemble DFT vs "PWSCF" DFT in metals
Stefano de Gironcoli
degironc at sissa.it
Fri Apr 17 10:24:13 CEST 2009
The functional is the same but the approach used to reach the solution
PWscf uses a self-consistent cycle technique while ensemble DFT recast
it in a global-minimization problem.
Ensemble DFT is guaranteed (if I remember correctly) to be a scheme
where the total energy decreases at each iteration while for the
self-consistent solution sometime the energy does not monotonically
decrease with the number of iteration before converging.
I think that the relative efficiency of the two methods may be different
for different systems and only experience can tell you wich one is more
efficient for your system.
Tricky systems are typically very anisotropic metallic slabs subjects
to charge sloshing (dealt with mixing_mode='local-TF' in PWscf) and/or
magnetic systems where many alternative solutions are close in energy
(larger smearing usually helps in stabilizing the system but may wash
out the physical effect... very many kpoints and a small smearing may be
In these cases the always decreasing nature of ensembe-DFT may be useful.
At variance with CP, PWscf exploits symmetry so for "crystals" PWscf may
I don't know how the many-k-points case, often needed for metals, is
dealt with in the ensemble DFT method... in principle all partially
occupied electronic states (nbnd*nks) should be treated together. But I
don't know how this is done in CP.
Methfessel and Paxton broadening may leads to non positive-definite
occupation numbers. This is usually not a big problem for PWscf , when
decent k-point sampling is used, but I think it is incompatible with the
ensemble DFT scheme. Luckily other efficient smearing techniques such
as the Marzari-Vanderbilt scheme are available that are positive-definite.
Stefano de Gironcoli - SISSA and DEMOCRITOS
Eduardo Ariel Menendez Proupin wrote:
> I am curious about the difference between the ensemble DFT method
> implemented in CP and the smeared DFT used in PWSCF with
> Methfessel-Paxton smearing. ¿What is better for what system?
> Eduardo Menendez
> Departamento de Fisica
> Facultad de Ciencias
> Universidad de Chile
> Phone: (56)(2)9787439
> URL: http://fisica.ciencias.uchile.cl/~emenendez
> Pw_forum mailing list
> Pw_forum at pwscf.org
More information about the users