The ABINIT Users Mailing List ( CLOSED )

[Matthieu Verstraete <mjv500@york.ac.uk>]

Hello Duckyoung, I have forwarded this to the forum as well. Probably will 
interest others. Also, others may be able to answer your questions. This 
really is what to forum is for, and you should send your queries there.

> As everybody does, I am calculating electronic strucutures with VASP or
> abinit or something else. The interesting thing with abinit is that I can
> calculate phonon dispersion w.r.t DFPT. But while I am doing that, I met 
> one question about smearing scheme.

> As far as I know, abinit use two smearing schemes, one is broading of
> kpoints meshing and the other one is finite temperature mimicking.
No: there are several smearing schemes, but they can serve 2 purposes, and 
act differently depending on the smearing function you use (occopt). The 
physical temperature is the FD smearing (occopt 3) and the others are 
intended to improve kpoint convergence. FD smearing also improves kpoint 
convergence.

> With my system, I am using occopt = 3 and use tsmear. In the manual, 
> abinit(or) says the default is 0.04 Ha for almost free electron type 
> metal such as Al. But the corresponding physical temperature is already 
> too high enough to melt the system away ( ~ 11000 K ). Even in your 
> paper, you have used 3000 K smearing which is above melting temperature 
> of Al. Thus I'd like to ask you what is the real meaning of tsmear = 0.04 
> Ha when occopt = 3. Could you make some analogy between Fermi-Dirac type 
> physical smearing value and "real system temperature"
No: the tsmear only gives the electron temperature. For fixed ions the 
crystal obviously won't melt. If you allow them to move (MD) you will find 
a transfer of energy to the ions, and it may melt if enough energy is 
transferred (the electrons are effectively on a thermostat, so T_e is 
constant and will keep feeding energy to the ions unless you give them a 
thermostat as well - see the different ionmov options). In a real 
equilibrated system the ions vibrate and the electron population has a FD 
distribution, both with the same temperature.

In the cases other than occopt 3, the smearing function decays quite 
quickly with energy, so high conduction band states are less occupied than 
in the FD case, and the system is closer to its 0K ground state. In 
particular for occopt 4 and 5 the cold smearing scheme is an intelligent 
way to ensure the total energy is as close as possible to the 0K value (to 
order tsmear^2 or ^3 I don't remember which). See Nicola Marzari's work 
cited in the PRB 65, 035111 (2001), in particular his thesis, for 
detailed explanations.


> Whenever somebody asks this question, the PRL 2006
which one? By the CEA gang? (hello boys!)

> (laser irradiation paper) is good example. They used the smearing more 
> than 6 eV. It is reasonable only when the observable time is less than 
> the time of electron -ion interaction. Am I right ?
That depends what you want to calculate: if you are looking for the 
effects of laser irradiation, you may want to see the energy transfer to 
the lattice, so you would start with excited electrons and then see where 
the energy goes. I'm not sure this is possible with abinit, unless you 
specify the occ by hand, and then you still need a method to evolve the 
occupation numbers, which is not coded. This has been done in the 
literature with other codes though.


> I am calculating a system with tsmear = 0.04 Ha.
even with cold smearing this is quite a high value (as you have said 
above) and will definitely not reflect the ground state properties.

> With less smearing value, the phonon goes to instable regime. Then, to 
> explain the instalbility and stability change w.r.t tsmear, I should know 
> the physical meaning of it. smaller would be better ?
The physical result is the limit for small smearing. In principle (as in 
the tutorial) you converge your kpoints for a given smearing, then 
decrease and converge kpoints again, until you get a value which is 
converged wrt tsmear as well. So I would say your phase is unstable. 
However you may be interested in a stabilizing effect due to electronic 
temperature. This is similar to Kohn anomalies or other T related 
instabilities, and could be due to the nesting of the Fermi Surface once 
the temperature is low enough and not too smeared out.

Usually people look at the opposite: the lattice can become unstable when 
the electronic population is excited. The phonons you are calculating 
actually come from second derivatives of the Free Energy, so the 
stabilizing term may come from the entropic contribution.

Another (somewhat related) issue is the true stabilization of phases at 
high temperature, but this is total temperature (ionic as well as 
electronic) and usually comes from lattice expansion as well. You should 
look at the full P/T diagram. There are some people in Uppsala and 
neighborhood who are professionals in this (Rajeev Ahuja is one I know, 
but also Borje Johansson e.g. PRL 98, 045503 (2007) for the pressure 
dependency). Just don't listen to them when they tell you to forget abinit 
and to use their LMTO codes :)

> or vice versa ? It was the motivation of the reason why I am interested 
> in the physical meaning of tsmearing. If you can suggest any other 
> literature concerning on this, I will appreciate you as well.
>
> --
> Duckyoung, Kim
> ph.D student
> Condensed Matter Theory Group
> Department of Physics, Uppsala University,
> Box-530, SE-75121 Uppsala, Sweden
> Phone : +46(0)-18-471 3567
> Fax     : +46(0)-18-471 3524


Matthieu