[Wien] Band Gap problem

[Rezaei Behrooz]

Dear all wien users

I have calculated band structure of GaN in wz phase under GGA(96)
approximation with band gap Eg=2.2eV,while the pseudopotential and
Experimental data show that it is equals to 3.5eV.Who can help me in
some simple terms that what is the reason of this main difference
between these calculations.Of course,I have calculated band structure of
GaN with other approximations( LSDA,GGA(91) ),that is not affect.
Thank you very much in advance.
best wishes,
================================================================
Behrooz Rezaei ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?Email: ***@tabrizu.ac.ir
Research Institute for Applied Physics and Astronomy
Tabriz University ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? phone: [phone]
Tabriz, Iran ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?Fax: [phone]
===================================================================

[Javad Hashemifar]

Post by Rezaei Behrooz
I have calculated band structure of GaN in wz phase under GGA(96)
approximation with band gap Eg=2.2eV,while the pseudopotential and
Experimental data show that it is equals to 3.5eV.Who can help me in
some simple terms that what is the reason of this main difference
between these calculations.Of course,I have calculated band structure of
GaN with other approximations( LSDA,GGA(91) ),that is not affect.

Dear Behrooz Rezaei,
I am not expert in this field, but it seems that usual LDA based
Exchange-correlation approximations are not appropriate for calculation of the
band gap of semiconductors. Peoples are developing some other approximations
like (Exact-Exchange method for better treatment of the band gap of
semiconductors).
In wien2k code, as far as I know, there is an option for exchange correlation
potential called Engel-Vosko approximation (PRB47, 13164) that seems to leads
to better band gaps rather than other approximations (LDA and GGA). In order to
use this approximations you have to use one of the switches 15, 25 or 26 as
VXC-SWITCH in case.in0 input file. I think switch 25 is better but I am not
sure. It would be nice if some of expert wien users help more about using this
exchange-correlation approximation.
Best regards,
Javad Hashemifar



=====
-------------------------------------------------------------------------------
Javad Hashemifar phone : ++49-30-8413 4804
Permanent : Physics Department, Isfahan University of Technology, Isfahan, Iran
Current : Fritz-Haber-Institut der MPG, Faradayweg 4-6, D-14195 Berlin, Germany
e-mails: ***@fhi-berlin.mpg.de & ***@ph.iut.ac.ir
-------------------------------------------------------------------------------



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[K. Ruebenbauer]

----- Original Message -----
From: Javad Hashemifar <***@yahoo.com>
To: <***@zeus.theochem.tuwien.ac.at>
Sent: Sunday, August 01, 2004 12:17 PM
Subject: Re: [Wien] Band Gap problem

Post by Javad Hashemifar

Post by Rezaei Behrooz
I have calculated band structure of GaN in wz phase under GGA(96)
approximation with band gap Eg=2.2eV,while the pseudopotential and
Experimental data show that it is equals to 3.5eV.Who can help me in
some simple terms that what is the reason of this main difference
between these calculations.Of course,I have calculated band structure of
GaN with other approximations( LSDA,GGA(91) ),that is not affect.

Dear Behrooz Rezaei,
I am not expert in this field, but it seems that usual LDA based
Exchange-correlation approximations are not appropriate for calculation of the
band gap of semiconductors. Peoples are developing some other

approximations

Post by Javad Hashemifar
like (Exact-Exchange method for better treatment of the band gap of
semiconductors).
In wien2k code, as far as I know, there is an option for exchange correlation
potential called Engel-Vosko approximation (PRB47, 13164) that seems to leads
to better band gaps rather than other approximations (LDA and GGA). In order to
use this approximations you have to use one of the switches 15, 25 or 26 as
VXC-SWITCH in case.in0 input file. I think switch 25 is better but I am not
sure. It would be nice if some of expert wien users help more about using this
exchange-correlation approximation.
Best regards,
Javad Hashemifar
=====
--------------------------------------------------------------------------

-----

Post by Javad Hashemifar
Javad Hashemifar phone : ++49-30-8413 4804
Permanent : Physics Department, Isfahan University of Technology, Isfahan, Iran
Current : Fritz-Haber-Institut der MPG, Faradayweg 4-6, D-14195 Berlin, Germany
--------------------------------------------------------------------------

-----

Post by Javad Hashemifar
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[K. Ruebenbauer]

----- Original Message -----
From: Rezaei Behrooz <***@tabrizu.ac.ir>
To: <***@zeus.theochem.tuwien.ac.at>
Sent: Saturday, July 31, 2004 5:54 AM
Subject: [Wien] Band Gap problem

Post by Rezaei Behrooz
Dear all wien users
I have calculated band structure of GaN in wz phase under GGA(96)
approximation with band gap Eg=2.2eV,while the pseudopotential and
Experimental data show that it is equals to 3.5eV.Who can help me in
some simple terms that what is the reason of this main difference
between these calculations.Of course,I have calculated band structure of
GaN with other approximations( LSDA,GGA(91) ),that is not affect.
Thank you very much in advance.
best wishes,
================================================================
Research Institute for Applied Physics and Astronomy
Tabriz University phone: [phone]
Tabriz, Iran Fax: [phone]
===================================================================
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Wien mailing list
http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien

[Naeini Ashkan]

Dear Behrooz

The Problem of electronic band gap is of more fundemental nature in DFT
approach. The reasen is that in the eigenvalues of Kohn-Sham equation have
nothing to do with the eigenvalues of schroedinger, except for the highest
occopied band, e.g. fermi niveau. In order to get the right values someone
needs to add quasi-particle correction terms likes GWA or RPA to consider
the quasi-particle nature of vacancies.

best regards,

Ashkan Naeini



-----Original Message-----
From: Rezaei Behrooz [mailto:***@tabrizu.ac.ir]
Sent: Saturday, July 31, 2004 5:54 AM
To: ***@zeus.theochem.tuwien.ac.at
Subject: [Wien] Band Gap problem


Dear all wien users

I have calculated band structure of GaN in wz phase under GGA(96)
approximation with band gap Eg=2.2eV,while the pseudopotential and
Experimental data show that it is equals to 3.5eV.Who can help me in
some simple terms that what is the reason of this main difference
between these calculations.Of course,I have calculated band structure of
GaN with other approximations( LSDA,GGA(91) ),that is not affect. Thank you
very much in advance. best wishes,
================================================================
Behrooz Rezaei ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?Email: ***@tabrizu.ac.ir
Research Institute for Applied Physics and Astronomy
Tabriz University ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? phone: [phone]
Tabriz, Iran ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?Fax: [phone]
===================================================================

[Naeini Ashkan]

Dear Behrooz

The Problem of electronic band gap is of more fundemental nature in DFT
approach. The reasen is that in the eigenvalues of Kohn-Sham equation have
nothing to do with the eigenvalues of schroedinger, except for the highest
occopied band, e.g. fermi niveau. In order to get the right values someone
needs to add quasi-particle correction terms likes GWA or RPA to consider
the quasi-particle nature of vacancies.

best regards,

Ashkan Naeini



-----Original Message-----
From: Rezaei Behrooz [mailto:***@tabrizu.ac.ir]
Sent: Saturday, July 31, 2004 5:54 AM
To: ***@zeus.theochem.tuwien.ac.at
Subject: [Wien] Band Gap problem


Dear all wien users

I have calculated band structure of GaN in wz phase under GGA(96)
approximation with band gap Eg=2.2eV,while the pseudopotential and
Experimental data show that it is equals to 3.5eV.Who can help me in
some simple terms that what is the reason of this main difference
between these calculations.Of course,I have calculated band structure of
GaN with other approximations( LSDA,GGA(91) ),that is not affect. Thank you
very much in advance. best wishes,
================================================================
Behrooz Rezaei ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?Email: ***@tabrizu.ac.ir
Research Institute for Applied Physics and Astronomy
Tabriz University ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? phone: [phone]
Tabriz, Iran ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?Fax: [phone]
===================================================================

[Yushan Wang]

Dear users,
while I was trying to carry out the LDA+U calculation, I found that the
scf procedure heavily oscilates as stated in the UG. therefore, I wanna to
use it in combination with the spin-orbital coupling. in the UG, it
recommends that " preferably running first LSDA+so and then slowly
switching on the LDA+O orbital field". who can give a detailed description
about how to operate in the calculation process.

thanks

[Stefaan Cottenier]

Post by Yushan Wang
while I was trying to carry out the LDA+U calculation, I found that the
scf procedure heavily oscilates as stated in the UG. therefore, I wanna to
use it in combination with the spin-orbital coupling. in the UG, it
recommends that " preferably running first LSDA+so and then slowly
switching on the LDA+O orbital field". who can give a detailed description
about how to operate in the calculation process.

First converge LDA+SO, as usual. Then take a small U (one tenth of what you
need), and a small mixing factor in case.inm (down to 0.0005 might be
necessary from time to time). Converge it, save and increase U in as much
steps as you need to arrive at the wanted U.

Stefaan