[Sax-project-users] Fwd: Re: GW calculation on organic semiconductor --1st

[Guido Roma]

Hello Layla and other users,

personally I had problems with the simple calculation of the bandgap of 
cubic silicon carbide (Layla, do you remember?),
I found a bandgap around 1.8 eV, only 0.4 eV larger than the LDA one.
After seeing this mail exchange I was wondering if the problem is really 
related to 1D systems or it can show up
also for a standard 3D semiconductor. By the way, if I do a COHSEX 
calculation I find very different results
(and so weird, as far as I understand ...). But maybe I am doing 
something wrong.

Guido


 
marsamos at democritos.it wrote:
> ----- Forwarded message from marsamos at democritos.it -----
>     Date: Mon, 20 Apr 2009 11:00:02 +0200
>     From: marsamos at democritos.it
> Reply-To: marsamos at democritos.it
> Subject: Re: [Sax-project-users] GW calculation on organic semiconductor --1st
>       To: xiaoyinxie <xiaoyinxie at 126.com>
>
> Dear user, we where already aware of such 1D problems! I perform some 
> calculations on PPV, polyacetylene and polyethylene and found that for 
> PPV the agreement with experiment is good instead PA and PE seems to 
> behave abnormaly, in the way you already noticed. If you try a 
> calculation with cohsex option instead of plasmonpole you will see that 
> the results are completely different. We suspect that the problem comes 
> from the way plasmon-pole is interpolating. In fact in strongly 
> anisotropic systems the traditional way of doing plasmon-pole is not 
> consistent. We are now working in a modification of the plasmon pole. I 
> hope that for august (beginning of september) we will release a new sax 
> version. But if you need the optical properties, i.e. to solve Bethe 
> Salpeter equation and you prefer to have the results before I suggest 
> to you to do the following:
>
> Perform a DFT calculation with exact-exchange (this really give good 
> results for the electronic structure, I test this for PA and PE). Then 
> calculate with sax the screening (Polarizability and W), and then 
> perform the Bethe-Salpeter calculation. This procedure is theoretically 
> correct with the lesser approximation. As EXX is still experimental 
> within espresso you will have to modify some things within the code (is 
> very easy). Let me now and I give you the exact computational procedure.
>
>
> Best regards
>
> Layla
>
>
> of this We are now studying polyacetylene  Quoting xiaoyinxie 
> <xiaoyinxie at 126.com>:
>
>   
>> Dear Sax developers:
>>   I am now doing GW calculation with Sax-1.0.0 , When I test the code 
>> with examples (example3)by calculating the band gap of Silicon ,It 
>> was running well, I got a reasonable bandgap of 0.98eV with 8*8*8 
>> k-mesh .
>>   Problem came out during I calculting the bandgap of P3HT 
>> <poly(3-hexylthiophene)> which has 1-D long polymer chain 
>> structure.As I increase the number of k-points ,The band gap 
>> decreases relatively .  A  0.26eV band gap of which experimental 
>> value is 1.9eV  was obtained with 12*1*1 k-mesh.
>>  Similar  problem happens when calculating the band gap of polyacetylene.
>>  I am not sure if I wrongly selected the parameters of GW calcultion 
>> or SAX code does not fit calculating band gap of oranic 
>> semiconductor(or some bugs exist) .
>>  Hope you can give me some recommendation.
>>  My input files are in the next Email titled <GW calculation on 
>> organic semiconductor --2st>
>>     
>
>
>
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