[Pw_forum] smaller band gaps

Thu Jun 25 13:28:59 CEST 2009

Thank you Prof. nicola, Lorenzo and Paolo.
 I got the point.

 Naol (Tamene)

On 6/24/09, pw_forum-request at pwscf.org <pw_forum-request at pwscf.org> wrote:
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> Today's Topics:
>
>    1. Re: smaller band gaps (Lorenzo Paulatto)
>    2. Re: Separating Raman modes (Paolo Giannozzi)
>    3. Re: smaller band gaps (Paolo Giannozzi)
>    4. Re: Separating Raman modes (Zubaer Hossain)
>    5. Re: Separating Raman modes (Paolo Giannozzi)
>    6. hybrid functional and Wannier functions (lan haiping)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Tue, 23 Jun 2009 17:16:22 +0200
> From: "Lorenzo Paulatto" <paulatto at sissa.it>
> Subject: Re: [Pw_forum] smaller band gaps
> To: "PWSCF Forum" <pw_forum at pwscf.org>
> Message-ID: <op.uvzgpku1a8x26q at paulax>
> Content-Type: text/plain; charset=utf-8; format=flowed; delsp=yes
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> In data 23 giugno 2009 alle ore 13:58:38, Naol Regassa
> <greatnaol at gmail.com> ha scritto:
>> My main concern is that DFT calculations reduce band gaps
>> by 50%.
>
> DFT does not predict band gap at all, for many materials it is related to
> the experimental one, but on assumption can be made a priori on its
> precision. Anyway, you are probably doing the calculation using the
> experimental lattice parameter, if you relax the system to the numerical
> equilibrium lattice parameter, you will probably get a better gap.
>
>> Does this mean that I can add this error to get the correct
>> band gap so that any body can understand it? How can one argue about
>> this condition?
>
> No, the fact that DFT does not predict the band gap is a well known
> problem of the theory, if you fix it with an empirical parameter it would
> be not only unethical, but also quite suspicious. If you want to have
> accurate band gap you have to use TDDFT or DFT+GW, which are both
> implemented in quantum espresso (I don't know the details of them, nor
> which is better for your specific purpose, you'll have to read a bit of
> literature).
>
> best regards
>
> P.S. if you are starting a new discussion in the mailing list, please
> write a new email, do not reply to a previous one
>
>
> --
> Lorenzo Paulatto
> SISSA  &  DEMOCRITOS (Trieste)
> phone: +39 040 3787 511
> skype: paulatz
> www:   http://people.sissa.it/~paulatto/
>
>      *** save italian brains ***
>   http://saveitalianbrains.wordpress.com/
>
>
> ------------------------------
>
> Message: 2
> Date: Tue, 23 Jun 2009 18:14:02 +0200
> From: Paolo Giannozzi <giannozz at democritos.it>
> Subject: Re: [Pw_forum] Separating Raman modes
> To: PWSCF Forum <pw_forum at pwscf.org>
> Message-ID: <4A40FF4A.4070108 at democritos.it>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> Stefano Baroni wrote:
>
>> The method by Mauri and coworkers
>> (whose implementation status in QE is unknown to me)
>
> it is implemented, but only for the "ordinary" phonon
> code (i.e. not for the Gamma-specific one) and only for
> norm-conserving pseudopotentials
>
> Paolo
> --
> Paolo Giannozzi, Democritos and University of Udine, Italy
>
>
> ------------------------------
>
> Message: 3
> Date: Tue, 23 Jun 2009 18:18:24 +0200
> From: Paolo Giannozzi <giannozz at democritos.it>
> Subject: Re: [Pw_forum] smaller band gaps
> To: PWSCF Forum <pw_forum at pwscf.org>
> Message-ID: <4A410050.9090100 at democritos.it>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> Lorenzo Paulatto wrote:
>
>> if you fix it with an empirical parameter it would be not only unethical
>
> Lorenzo, take it easy: there is nothing "ethically" wrong in trying
> to adjust the gap, as long as it is clearly stated
>
> P.
> --
> Paolo Giannozzi, Democritos and University of Udine, Italy
>
>
> ------------------------------
>
> Message: 4
> Date: Tue, 23 Jun 2009 11:28:30 -0500
> From: Zubaer Hossain <zubaexy at gmail.com>
> Subject: Re: [Pw_forum] Separating Raman modes
> To: PWSCF Forum <pw_forum at pwscf.org>
> Message-ID:
> 	<51ec861d0906230928p791f99a7yb86b747d6ad5facd at mail.gmail.com>
> Content-Type: text/plain; charset="iso-8859-1"
>
> How about decomposing eigenstates into that for the individual atoms and
> summing them for each species, since the phonon frequencies are computed
> already, and the eigenmodes for each frequency are known. By analyzing these
> quantities graphically for each species, optical frequencies of the main
> three modes could be detected, I think.
>
> Zubaer
> Department of Mechanical Science and Engineering
> Univ. of Illinois at Urbana-Champaign, USA
>
> On Tue, Jun 23, 2009 at 11:14 AM, Paolo Giannozzi
> <giannozz at democritos.it>wrote:
>
>> Stefano Baroni wrote:
>>
>> > The method by Mauri and coworkers
>> > (whose implementation status in QE is unknown to me)
>>
>> it is implemented, but only for the "ordinary" phonon
>> code (i.e. not for the Gamma-specific one) and only for
>> norm-conserving pseudopotentials
>>
>> Paolo
>> --
>> Paolo Giannozzi, Democritos and University of Udine, Italy
>> _______________________________________________
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> Message: 5
> Date: Tue, 23 Jun 2009 18:35:42 +0200
> From: Paolo Giannozzi <giannozz at democritos.it>
> Subject: Re: [Pw_forum] Separating Raman modes
> To: PWSCF Forum <pw_forum at pwscf.org>
> Message-ID: <4A41045E.5060708 at democritos.it>
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>
> Zubaer Hossain wrote:
>
>> How about decomposing eigenstates into that for the individual atoms and
>> summing them for each species, since the phonon frequencies are computed
>> already, and the eigenmodes for each frequency are known. By analyzing
>> these quantities graphically for each species, optical frequencies of
>> the main three modes could be detected, I think.
>
> years ago we did something similar:
>     "Phonon Dispersions in Ga(x)Al(1-x)As Alloys", PRL 65, 84 (1990).
> Alloy modes were projected over crystal modes. In that particular
> case the "mass approximation" was giving an accurate and convenient
> way to calculate the dynamical matrix in relatively large supercells
>
> Paolo
> --
> Paolo Giannozzi, Democritos and University of Udine, Italy
>
>
> ------------------------------
>
> Message: 6
> Date: Wed, 24 Jun 2009 02:05:09 +0800
> From: lan haiping <lanhaiping at gmail.com>
> Subject: [Pw_forum] hybrid functional and Wannier functions
> To: PWSCF Forum <pw_forum at pwscf.org>
> Message-ID:
> 	<c92002fa0906231105l7b5918aeh44848111cbe1b20b at mail.gmail.com>
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>
> Dear All:
>
>  I want to study  electronic structures with hybrid functional.  Is it
> possible for us to using wannier90 package to analyse related bands at
> present version ?  Such as  WFs and  bands structure.
>
>
> Regards,
>
> Hai-Ping
>
>
>
> --
> Hai-Ping Lan
> Department of Electronics ,
> Peking University , Bejing, 100871
> lanhaiping at gmail.com, hplan at pku.edu.cn
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