Dear Prof. Marzari, Thanks a lot for your invaluable response. Regards CMI Okoye
*------------------------------Chukwuemeka M I Okoye* *Department of Physics and Astronomy,* *University of Nigeria, * *Nsukka, Enugu State,* *Nigeria* Telephone: +234 7038766990 E-mail: [email protected] On Thu, Jul 6, 2017 at 4:19 PM, Nicola Marzari <[email protected]> wrote: > 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 >
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