[Wien] Formation energy + some other querries!
rcais@cal3.vsnl.net.in
rcais at cal3.vsnl.net.in
Wed Feb 6 05:28:07 CET 2008
For an alloy, formation energy energy is what is conventionally used. Cohesive energy could also be used for an initial assessment. The calculation would be as follows :
Formation energy for GaN with Mn(Ga15,Mn1,N16) : total ENE of Ga15MnN16 - 15*total ENE for Ga metal in standard state structure - 1* total ENE for Mn metal in standard state structure - 16* total ENE for N in standard structure. Subtracting individual atom energies in weighted amounts from the total energies gives the same result since they cancel out. A positive value suggests a phase that is unstable / metastable) with respect to the metals. Very low negative or positive values of cohesive energy would indicate a definitely unstable phase whereas metastable existence is possible otherwise.
Please correct me if I am wrong.
Chandrika
Anjali :By zero energy do you mean zero point energy?
----- Original Message -----
From: Stefaan Cottenier <Stefaan.Cottenier at fys.kuleuven.be>
Date: Tuesday, February 5, 2008 7:08 pm
Subject: Re: [Wien] Formation energy + some other querries!
To: wien at zeus.theochem.tuwien.ac.at
>
>
> Probably two issues are being confused here. The zero on the energy
>
> scale *for the eigenvalues* is defined in wien2k as the average of
> the
> potential in the interstitial. That is an arbitrary convention, and
>
> this zero is needed to express the Fermi energy and to plot the
> DOS.
> However, what you need in order to calculate formation energies is
> the
> *total energy* (:ENE) and not the eigenvalues. The total energy in
> an
> all electron code has a well-defined zero (in contrast to the
> energy
> in a pseudopotential calculation). For instance, if for a
> particular
> unit cell :ENE=-7345.730289 Ry, it means you need that amount of
> energy to bring all electrons and all nuclei in the unit cell to
> infinite distances from each other. That situation -- all particles
> at
> infinite distances -- corresponds to zero. Exactly the same as in
> the
> H-atom, where you need exactly 1 Ry (13.6 eV) to bring proton and
> electron at infinite distance from each other (at zero speed).
>
> Moreover, regarding the formula you quote, one can debate whether
> it
> makes more sense to use the total energy for a Mn and Ga free atom
> (=calculate cohesive energy) or the total energy for bulk Mn and
> bulk
> Ga (=calculate formation energy). If the bulk phases are known to
> exist, the formation energy is probably what you need (==> the
> alloy
> will possible split into bulk phases, which are more stable than
> the
> gas phase). If one of the constituents is a very dilute impurity,
> I'm
> not sure which of both expression is most meaningful. Anyone a
> comment?
> Stefaan
>
>
> Quoting Nandan Tandon <nandan.tandon at gmail.com>:
>
> > Dear Prof Blaha
> >
> > I am still not convinced that the zero of energy is not required.
> >
> > When we are considering the differences in the single particle
> energies> for a particular system, I agree that the arbitrary
> number representing
> > the zero is not required since it cancels but for differences in
> total> energies of different systems will require the zero reference.
> >
> > I will explain my problem in detail :
> >
> > I wish to find the formation energy for a Mn impurity in GaN. I
> do a
> > supercell calculation (32 atom per unit cell) for pure GaN and Mn
> > substituted GaN. I also calculate the energy of a single Mn atom
> and a
> > single Ga atom by putting in a large unit cell. Then the
> formation energy
> > is
> >
> > FE = E(Ga_16N_16) + E(Mn) - E(Ga)- E(Ga_15MnN_16)
> >
> > Each of the above total energies are with respect to the
> respective zero
> > energy for that system. Is it that when wien gives the total
> energy per
> > unit cell for any system, it takes care of the reference level?
> >
> > Regards
> >
> > Anjali Kshirsagar
> >
> > On 2/5/08, Peter Blaha <pblaha at theochem.tuwien.ac.at> wrote:
> >> > 1. We need to calculate the Formation energies for impurities
> in some
> >> > semiconductor material. Thus an algebra with the total
> energies
> >> of different
> >> > systems. I therefore need to know the zero of my energy. I am
> not
> >> able to find
> >> > where this value is written. Should be in case.scf0. Can you
> >> please tell us
> >> > where to look for average interstitial potential or the zero
> of
> >> energy for a
> >> > particular case?
> >>
> >> I don't think you need to know the zero of your energy. It is an
>
> >> arbitrary number.
> >>
> >> > 2. This problem we are facing with the 06, 07 versions.
> Earlier,
> >> there was a
> >> > provision in case.in1 file that unit could be specified and
> >> accordingly the
> >> > k-list will be read from in1 or case.klist file. Now the
> program
> >> does not allow
> >> > us to read the k-list from in1. Why such a change has been
> made?
> >> Actually I
> >> > find that there is a rewind command for the specific unit and
> if
> >> it is unit 5,
> >> > case.in1 is rewind and there is an error. I did not see any
> such
> >> problem on the
> >> > mailing list. We also posted the question but there was no
> reply
> >> even after
> >> > putting it 2-3 times. Hence I want to knwo if we are making
> any
> >> error or is
> >> > there a problem. I will accordingly send you the actual files
> and
> >> the error we
> >> > are getting.
> >>
> >> Using unit=5 for the k-vector list is not recommended. It should
>
> >> still be possible,
> >> but you need an extra line before the k-list (put 1.d-15) and
> the
> >> k-list must be in the new
> >> format (4I10 instead of 4I5, check the new klist format).
> >> Unfortunately the UG has not been updated properly.
> >>
> >> > 3. We are also not able to run self-consistency for Monkhost
> mesh/ special
> >> > k-points? Please guide us where to get the information
> regarding
> >> this. We are
> >> > doing slab calculations and for the large supercell we wish to
>
> >> use Monkhost
> >> > mesh for reducing the computations.
> >>
> >> You can put ANY k-mesh into case.klist; specify TEMP (or GAUSS)
> for
> >> the Fermi-method
> >> and run an scf-cycle. (KGEN produces a special point mesh
> anyway,
> >> only for centered
> >> lattices it is different due to symmetry constrains).
> >>
> >> --
> >>
> >> P.Blaha
> >> -----------------------------------------------------------------
> ---------
> >> Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
> >> Phone: +43-1-58801-15671 FAX: +43-1-58801-15698
> >> Email: blaha at theochem.tuwien.ac.at WWW:
> >> http://info.tuwien.ac.at/theochem/
> >> -----------------------------------------------------------------
> ---------
> >> _______________________________________________
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>
>
> --
> Stefaan Cottenier
> Computational Materials Engineering (CME) &
> Center for Computational Engineering Science (CCES)
> RWTH Aachen University
> Mauerstrasse 5
> DE-52064 Aachen
> Germany
>
> e-mail: stefaan.cottenier at fys.kuleuven.be (preferred)
> cottenier at ghi.rwth-aachen.de (avoid)
>
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