[Wien] Formation energy + some other querries!

Thu Feb 7 07:47:22 CET 2008

Dear Chandrika, Stefaan, Prof. Blaha,

Thank you for your replies. Your mail have clarified few points very nicely.

Regards,

Nandan.

On 2/7/08, Chandrika <rcais at cal3.vsnl.net.in> wrote:
> You cannot do gaseous molecules with Wien. Since these are ground state (0
> K) energies you need to compute energy of crystalline N. Your formation
> energy value will also be for the ground state. For real temperatures
> vibrational components will be needed.
> Best wishes
> Chandrika
> ----- Original Message -----
> From: "Nandan Tandon" <nandan.tandon at gmail.com>
> To: "A Mailing list for WIEN2k users" <wien at zeus.theochem.tuwien.ac.at>
> Sent: Wednesday, February 06, 2008 4:46 PM
> Subject: Re: [Wien] Formation energy + some other querries!
>
>
> > By zero energy, we really meant the zero of the energy scale.
> >
> > Now i understand that the formation energy can be calculated as:
> > 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.
> >
> > Since in this formula the total energy is with respect to the same
> > zero, (as Stefaan has indicated), this formula should work provided
> > all the total energies are available.
> > This leads to the question of how to evaluate total energy for N in
> > standard structure since N would be present only in the gaseous
> > phase??????
> >
> > Also, just to confirm what Stefaan has said, if one does an atomic
> > calculation for H, the total energy evaluated should be around 1Ryd?
> >
> > Regards,
> >
> > Nandan.
> >
> > On 2/6/08, rcais at cal3.vsnl.net.in <rcais at cal3.vsnl.net.in> wrote:
> >> 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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> >> > >>
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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)
> >> >
> >> >
> >> > Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm
> >> >
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