Hi, Karen,

There are several facets in this issue:

* When a finite temperature (smearing) is used just for convergence 
acceleration, the free energy is a 
computational artifact that is formally needed to restore the variational 
property. Tests for the size of the smearing and
for the fineness of the k-point grid should be carried out to monitor the 

* A given client code might be interested, rather than in the free energy, in 
the zero-temperature (extrapolation to zero-smearing) E_tot, rather than in the 
free energy. Depending on the type of smearing, E_tot and E_free deviate from 
each other: the difference is quadratic in the smearing for Fermi-Dirac
smearing, but much smaller for Methfessel-Paxton or for the cold-smearing of 
Marzari and Vanderbilt. (See the article by Kresse and Furthmuller [1])

* There is only one slot for the energy in the i-PI protocol specification, and 
it is not clear what that energy should be.

Depending on your use case, you might want to use MP or cold smearing. There 
could also be an enhancement of the i-PI protocol to negotiate the kind of 
'energy' to be sent. Maybe an fdf flag in Siesta could be used to select what 
is sent through the interface. What do other codes send through the interface?

  Best regards,


[1] Kresse, G., and J. Furthmüller. 1996. “Efficiency of Ab-Initio Total Energy 
Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set.” 
Computational Materials Science 6 (1): 15–50. 

----- El 2 de Noviembre de 2021, a las 22:22, Karen Fidanyan 
karen.fidan...@mpsd.mpg.de escribió:

| Dear Siesta users,
| I noticed that when communicating with a Master code, e.g. via i-PI
| socket, the values that would be sent to a socket are
| the forces and the _total_ energy, not the _free_ energy, even though I
| use Fermi-Dirac smearing.
| At the same time, in the manual I read: "We finally note that, in both
| cases (Fermi-Dirac and Methfessel-Paxton), once a finite temperature has
| been chosen, the relevant energy is not the Kohn-Sham energy, but the
| Free energy. In particular, the atomic forces are derivatives of the
| Free energy, not the KS energy."
| This means, to my understanding, that a client code receives an energy
| and forces that are inconsistent to each other, and the energy is
| somewhat ill-defined. Do you know why such choice was made? Is there any
| problem with sending the free energy?
| Many thanks,
| Karen Fidanyan
| PhD student
| Max Planck Institute for the Structure and Dynamics of Matter
| Hamburg, Germany
| --
| SIESTA is supported by the Spanish Research Agency (AEI) and by the European
| H2020 MaX Centre of Excellence (http://www.max-centre.eu/)
SIESTA is supported by the Spanish Research Agency (AEI) and by the European 
H2020 MaX Centre of Excellence (http://www.max-centre.eu/)

Responder a