[Pw_forum] TDDFT approach for periodic system

[Stefano Baroni]

Just a couple of further comments.
1) The ability (or lack thereof) of (adiabatic) tddft to properly describe optical excitations in extended systems does not depend on the code's ability to sample the whole BZ using special k-points. The current version of the tddfpt code distributed with QE is only apt to sample k=0 Bloch state, but periodic systems can nevertheless be treated (admittedly not very efficiently) to arbitrary numerical accuracy, by using suitable supercells.
2) adiabatic XC functionals miss excitonic effects in periodic systems (irrespective of the number of k-points used to sample the BZ) for the same reason why they miss Rydberg series in molecular systems. Always be suspicious of statements such as "TDDFT works for molecules, but it fails for crystals". This is nonsense: any crystal is nothing but a "big molecule". TDDFT works for both, and it fails for both. Only the failures manifest themselves in different ways, according to the size of the system.
Cheers - SB


On Apr 20, 2011, at 11:21 AM, Giuseppe Mattioli wrote:

> Dear all
> I did not say that TDDFPT does not work with periodic system, but you cannot 
> have a realistic absorption spectra of a slab (or bulk crystal) if you sample 
> the Brillouin zone by using Gamma only... TDDFPT works fine with finite 
> systems, i.e., which do not show a (strong) k dependence of the psi_n,k 
> eigenvalues.
> 
> Giuseppe
> 
> On Wednesday 20 April 2011 03:00:41 Johari, Priya Sudhir wrote:
>> Thanks Marton, its working.
>> 
>> On Tue, Apr 19, 2011 at 5:40 PM, Marci <vormar at gmail.com> wrote:
>>> Dear Priya,
>>> 
>>> Though I have never tried, it should work with periodic systems if you
>>> sample the Brillouin zone only with the gamma point. Keep in mind that
>>> the usual adiabatic approximations with (semi)local functionals might
>>> not work very well for calculating the absorption of periodic systems.
>>> 
>>> Marton
>>> 
>>> --
>>> Marton Voros
>>> PhD student
>>> Department of Atomic Physics
>>> Budapest University of Technology and Economics
>>> Budafoki út 8., H-1111, Budapest, Hungary
>>> www.fat.bme.hu/MartonVoros
>>> 
>>> On Tue, Apr 19, 2011 at 11:02 PM, Johari, Priya Sudhir
>>> 
>>> <priya_johari at brown.edu> wrote:
>>>> Thanks Duy, but the link does not give sufficient information. However,
>>>> I found a discussion on forum's archive itself where Giuseppe Mattioli
>>>> has mentioned that TDDPFT does not work for periodic systems :-(
>>>> 
>>>> Dr Priya Johari.
>>>> Brown University,
>>>> Providence, RI - 02906. USA
>>>> 
>>>> On Tue, Apr 19, 2011 at 3:59 PM, Duy Le <ttduyle at gmail.com> wrote:
>>>>> If you do a little homework (such as googling keyword TD-DFPT), you'll
>>>>> find this link
>>>>> http://qe-forge.org/projects/tddfpt/
>>>>> I have not read it carefully but I guess it contains some useful
>>>>> information.
>>>>> --------------------------------------------------
>>>>> Duy Le
>>>>> PhD Student
>>>>> Department of Physics
>>>>> University of Central Florida.
>>>>> 
>>>>> "Men don't need hand to do things"
>>>>> 
>>>>> 
>>>>> 
>>>>> On Tue, Apr 19, 2011 at 3:26 PM, Johari, Priya Sudhir
>>>>> 
>>>>> <priya_johari at brown.edu> wrote:
>>>>>> TD-DFPT
>>>>> 
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> 
> 
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Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
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