[Thermo_pw-forum] nonstoichiometric compounds

[Andrea Dal Corso]

On Sat, 2016-08-13 at 13:16 +0200, Gunnar Palsson wrote:
> Dear Andrea, 
> 
> First I would like to tell you that I find your thermo_pw package extremely useful in many ways. I was inspired by your article J. Phys. Condensed Matter 26, 335401 (2014) and have been practising using your PS library v1.0 and the PAW potentials, for pure vanadium and niobium to calculate the thermoelastic properties. I have been thinking for quite a while about calculating the thermophysical properties and changes in free energy of metal hydrides, specifically vanadium hydride, in an attempt to determine the phase boundaries. I am also interested in the changes in the elastic constants with temperature and hydrogen. The question I have is whether one can use thermo_pw to calculate the Helmholtz free energy in the quasi harmonic approximation as a function of temperature for a supercell of vanadium with few hydrogen atoms. The size of the supercell determines what part of the phase diagram one can access in terms of concentration. So for 16 vanadium atoms, one can go from 1/
>  16 to 16/16 in H/V concentration for example. It is my understanding that this requires ibrav=0, which may not be fully implemented? If not it would still be valuable to look into the stoichiometric hydride, V2H, and VH. The V2H belongs to space group 141. Finally, I have some neutron scattering data on hydrogen containing vanadium, and it would be extremely valuable to use the output of thermo_pw to calculate various neutron scattering cross sections and compare with experiment, which I can probably do. 
> 
Using ibrav=0 thermo_pw cannot use symmetry to reduce the number of
tensor components that it calculates. Moreover it does not know the
Bravais lattice number and presently it cannot calculate it, so it
cannot find the automatic path, you have to provide it from input.
I think it might also be possible to use ibrav=0 for phonon calculations
and for the calculation of harmonic thermal properties, but all the
rest, such as the anharmonic properties is most probably uncorrect. In
any case I have not worked on this option so far, so I think it is very
probable that there are problems. 

Andrea

> Best regards,
> Dr. Gunnar Palsson,
> Researcher,
> Uppsala University.
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Andrea Dal Corso                    Tel. 0039-040-3787428
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