[Pw_forum] Comparison of total energies

[Nicola Marzari]

On 06/07/2017 16:53, Chukwuemeka Okoye wrote:
> Dear Prof. Nicola Marzari,
> 
> Thank you so much for your suggestions.
> 
> At the moment, I am actually interested in comparing the same 1: 1: 1: 1 
> stoichiometry for the quaternary compound, but different  atom 
> arrangements e.g.Type-I: 1 A atom(0,0,0), 1 B atom(.5,.5,.5), 1 C 
> atom(0.25,0.25,0.25), 1 D atom(0.75,0.75,0.75)
> 
> Type-II: 1 A atom(0,0,0), 1 B atom(0.25,0.25,0.25), 1 C 
> atom(0.5,0.5,0.5), 1 D atom(0.75,0.75,0.75)
> 
> Type-III: 1 A atom(0,0,0), 1 B atom(0.25,0.25,0.25), 1 C 
> atom(0.75,0.75,0.75), 1 D atom(0.5,0.5,0.5) .
> 
> These calculations are performed with the same pseudo-potentials.
> 
> Really, my worry is on the relevance of the total energies in 
> determining the most stable configuration of atomic arrangement since 
> Quantum Espresso is not an all-electron method.
> 
> Thanks


I think you'll be fine, but read the literature I sent. You raise
several questions?

1) is QE able to reproduce all-electron
results. The answer is yes, broadly speaking, and especially I would say
for relative changes in the formation energies (rather than the absolute 
ones). Not to mention that all-electron results with one code do not
agree perfectly with another code - so there is no truth. Read:
https://molmod.ugent.be/deltacodesdft

2) is PBE, or PBESOL, or whatever xc potential, good enough for the 
thermodynamics of solids? Mostly yes, but not always. But typically good 
enough for your problem, since you are asking relative rankings of very 
similar systems.

3) are your chosen pseudopotentials good? my suggestion is for 
http://materialscloud.org/sssp, remembering to cite who made the
original pseudos

4) remember also that you are calculating total energies at 0K; would 
the free energies at 500K be ordered in the same way? probably yes,
for your case, but it's relevant for systems where e.g. configurational
entropy is large (alloys).

nicola

----------------------------------------------------------------------
Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL
Director, National Centre for Competence in Research NCCR MARVEL, EPFL
http://theossrv1.epfl.ch/Main/Contact http://nccr-marvel.ch/en/project