Thank You very much Dear Prof. <span name="Burak Himmetoglu" class="gD">Burak, Prof Giuseppe</span> and Dear Prof. Matteo for giving such a explanation for my problem. Thank you very much once again to all my DEAR eminent Prof. for your kind attention and help.<br>
<br><div class="gmail_quote">On Mon, Oct 8, 2012 at 2:54 PM, Giuseppe Mattioli <span dir="ltr"><<a href="mailto:giuseppe.mattioli@ism.cnr.it" target="_blank">giuseppe.mattioli@ism.cnr.it</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div class="HOEnZb"><div class="h5"><br>
<br>
Dear all<br>
<br>
Burak has indeed described well the problem, but something more can be<br>
said: the Ga 3d shell (as well as the Zn 3d shell in ZnO) is almost<br>
atomic-like and give rise to a very narrow band in III-V<br>
semiconductors, horribly affected by delocalisation (aka<br>
self-interaction) problems. This band is raised in energy far more<br>
than the As (3p) valence band; so the two bands are mixed (and they<br>
should not) and the valence band maximum is pushed up towards the<br>
conduction band minimum. Moreover, you cannot extract meaningful<br>
self-consistent U values by linear response procedures in the limit of<br>
a simplified "on-site, one-shell" approach. However, it is not<br>
forbidden to use "semi-empirical" U values, as long as you have a good<br>
knowledge of the physics of your system... The above problems in DFT<br>
calculations of GaAs and ZnO are addressed in a Zunger paper, very<br>
long and quite... difficult to flick through.<br>
<br>
Lany, S. and Zunger, A.; Phys. Rev. B 78, 235104 ͑(2008͒).<br>
<br>
In a short, you may try to decouple the Ga (3d) band from the As (3p)<br>
band by hammering with some empirical U value the head of the Ga 3d<br>
band. And you can recover a very good band gap value by pushing down<br>
(more softly...) the As 3p band. You may test your results against a<br>
hybrid functional. I've done it in the case of ZnO, and I've obtained<br>
quite good results.<br>
<br>
Matteo suggest that the Ga 3d has to be included in the calculation.<br>
In my experience (but I'm the "empirical" kind of computational<br>
guy...) The Ga 3d shell is mandatory in the case of GaN if you want a<br>
good lattice parameter. In the case of GaAs you can obtain good<br>
results (band gap included) by embedding the Ga 3d electrons within<br>
the Ga pseudopotential, and by using a NLCC approach.<br>
This leads to some kind of decoupling of the Ga 3d and As 3p bands. I<br>
remember some old papers (2000-2005) by Van de Walle and Neugebauer on<br>
GaN in which a Ga PP without 3d shell was used after a lattice<br>
parameter optimisation performed by using a Ga PP with 3d shell.<br>
<br>
HTH<br>
<br>
Giuseppe<br>
<br>
Quoting Burak Himmetoglu <<a href="mailto:himm0013@umn.edu">himm0013@umn.edu</a>>:<br>
<br>
> Dear Brahma,<br>
><br>
> Ga has a full d-shell. Therefore, if you try to perturb the d-shell of Ga,<br>
> you can reduce the number of electrons localized on d-states by a positive<br>
> Hubbard_alpha but there is no way to increase their occupations with a<br>
> negative Hubbard_alpha. Therefore, the response matrices will be non-linear<br>
> and you will get very small numbers in them. When you invert these<br>
> matrices, very large numbers will appear.<br>
><br>
> Calculating U for full d-shells is not easy, and there is no simple<br>
> solution (Another known example is ZnO). My personal opinion is that the<br>
> use of LDA+U is questionable for filled d-shells. In fact if you check the<br>
> paper mentioned by Matteo, the gap for GaAs is corrected by applying U and<br>
> V (inter-site interactions) on Ga-p states and states which overlap with<br>
> Ga-p. No correction on Ga-d is applied.<br>
><br>
> Best regards,<br>
><br>
> Burak<br>
><br>
> On Sun, Oct 7, 2012 at 12:02 PM, Bramha Pandey<br>
> <<a href="mailto:pandey.bramha@gmail.com">pandey.bramha@gmail.com</a>>wrote:<br>
><br>
>> sorry for wrong matrix copied in previous mail.<br>
>><br>
>> CHI0^-1 - CHI^-1 Matrix<br>
>><br>
>> 0.0000 0.0000 0.0000<br>
>> 0.0000 0.0000 0.0000<br>
>><br>
>> 0.0000 0.0000 0.0000<br>
>> type: 1 U0 = 0.000000000000000E+000<br>
>><br>
>> On Sun, Oct 7, 2012 at 10:30 PM, Bramha Pandey<br>
>> <<a href="mailto:pandey.bramha@gmail.com">pandey.bramha@gmail.com</a>>wrote:<br>
>><br>
>>> Dear Prof. Matteo,<br>
>>> i am using Ga, pseudo with 3d as a valence, Hubbard_l=2 is set for 'Ga'.<br>
>>> If it should change how much value i have to set it?<br>
>>><br>
>>> Dear sir i am totally struck by zero matrix element in ch0^-1 - chi^-1,<br>
>>> so always it gifted me ZERO value where as ch0 and chi matrix are NON ZERO<br>
>>> with same element value.<br>
>>> CHI_0 Matrix<br>
>>> -0.0002 -0.0002 0.0005<br>
>>> -0.0002 -0.0002 0.0005<br>
>>> 0.0005 0.0005 -0.0010<br>
>>> CHI Matrix<br>
>>> -0.0002 -0.0002 0.0005<br>
>>> -0.0002 -0.0002 0.0005<br>
>>> 0.0005 0.0005 -0.0010<br>
>>><br>
>>> CHI0^-1 - CHI^-1 Matrix<br>
>>><br>
>>> 0.0000 0.0000<br>
>>> 0.0000 0.0000<br>
>>> type: 1 U0 = 0.000000000000000E+000<br>
>>> This above is for all the 2 atoms supercell combination when we take the<br>
>>> ZB structure with 2 atoms: (1) when both atoms are 'Ga'<br>
>>> (2) when both atoms are 'Ga1' and 'Ga' (3) when both atoms are different<br>
>>> 'Ga' and 'As'.<br>
>>><br>
>>> All above three conditions are tested under one atom(Ga1 or Ga) is<br>
>>> perturbed.<br>
>>> Please Sir give me some ray of hope so that i can get rid of this<br>
>>> situation.<br>
>>><br>
>>><br>
>>> On Sun, Oct 7, 2012 at 8:10 PM, Matteo Cococcioni <<a href="mailto:matteo@umn.edu">matteo@umn.edu</a>> wrote:<br>
>>><br>
>>>> Dear Bramha,<br>
>>>><br>
>>>> in J. Phys.: Condens. Matter 22 (2010) 055602 we did a similar<br>
>>>> calculation but we got pretty different numbers (see Table 2 of the<br>
>>>> reference). Maybe the states of Ga you applied U or alpha to are<br>
>>>> completely full. what is the value of l set for Ga in set_hubbard_l ?<br>
>>>> did you change that? also, for Ga, you better use a pseudo with 3d<br>
>>>> states in the valence rather than frozen in the core.<br>
>>>><br>
>>>> Matteo<br>
>>>><br>
>>>><br>
>>>><br>
>>>> On Sun, Oct 7, 2012 at 7:31 AM, Bramha Pandey <<a href="mailto:pandey.bramha@gmail.com">pandey.bramha@gmail.com</a>><br>
>>>> wrote:<br>
>>>> > First of all thank you very much Dear Prof.Stefano.<br>
>>>> > I have change the input as ZB 'GaAs' structure and calculate the 'U'<br>
>>>> value<br>
>>>> > for both 'Ga' and 'As' which is given bellow.<br>
>>>> > But i wondered to see the 'U' value of 'Ga' 1010.79 which is quite<br>
>>>> large so<br>
>>>> > how can i figure out this is correct or wrong?<br>
>>>> > No of Atom U_Ga U_As<br>
>>>> > 2 1043.35792360332 2.87817590827985<br>
>>>> > 16 1010.81003743078 2.82970777976411<br>
>>>> > 54 1010.79529573193 2.82968584264055<br>
>>>> > 128 1010.79528904780 2.82968583269392<br>
>>>> > 250 1010.79528904476 2.82968583268940<br>
>>>> > Any comment is welcome.<br>
>>>> > --<br>
>>>> > Thanks and Regards<br>
>>>> > Bramha Prasad Pandey<br>
>>>> > Ph.D Student Indian School of Mines(ISM)<br>
>>>> > Dhanbad, INDIA.<br>
>>>> ><br>
>>>> ><br>
>>>> > _______________________________________________<br>
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>>>><br>
>>>><br>
>>>><br>
>>>> --<br>
>>>> Matteo Cococcioni<br>
>>>> Department of Chemical Engineering and Materials Science,<br>
>>>> University of Minnesota<br>
>>>> 421 Washington Av. SE<br>
>>>> Minneapolis, MN 55455<br>
>>>> Tel. +1 612 624 9056 Fax +1 612 626 7246<br>
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>>>><br>
>>><br>
>>><br>
>>><br>
>>> --<br>
>>> Thanks and Regards<br>
>>> Bramha Prasad Pandey<br>
>>> Ph.D Student Indian School of Mines(ISM)<br>
>>> Dhanbad, INDIA.<br>
>>><br>
>>><br>
>><br>
>><br>
>> --<br>
>> Thanks and Regards<br>
>> Bramha Prasad Pandey<br>
>> Ph.D Student Indian School of Mines(ISM)<br>
>> Dhanbad, INDIA.<br>
>><br>
>><br>
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>><br>
><br>
<br>
Giuseppe Mattioli<br>
ISM-CNR<br>
Italy<br>
<br>
<br>
<br>
<br>
<br>
<br>
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</div></div></blockquote></div><br><br clear="all"><br>-- <br>Thanks and Regards<br>Bramha Prasad Pandey<br>Ph.D Student Indian School of Mines(ISM)<br>Dhanbad, INDIA.<br><br>