[QE-users] epsilon.x and hybrids

[Giuseppe Mattioli]

Dear all
I don't want to raise the confusion level, so please correct me if I'm  
wrong... If you want to calculate a heavily approximate absorption  
spectrum of a (large and non-symmetrical) periodic system after a  
ground state hybrid calculation, there used to be an option in the  
turbo_davidson.x and turbo_lanczos.x codes, namely no_hxc=.true.,  
which permits an independent-electron calculation. At least, hybrid  
functionals and Gamma ground states should be properly treated by such  
codes, resulting in an absorption spectrum compatible with those  
obtained by using epsilon.x, which, AFAIK, calculates  
<occupied|r|virtual> contributions to the absorption spectrum.  
However, I don't know how much this kind of calculation is expensive  
for large supercells. Of course if you are not interested in  
absorption, then my suggestion is nonsense...
HTH
Giuseppe

Quoting Lorenzo Paulatto <paulatz at gmail.com>:

> Also, epsilon.x cannot use symmetry-reduced grids, which would be a  
> huge wast of time with hybrids, but you can use open_grid.x after  
> the pw.x calculation too obtain the full grid and work around this  
> problem.
>
> cheers
>
> On 4/8/20 6:38 PM, Paolo Giannozzi wrote:
>> I think epsilon.x assumes that the dipole element of x can be  
>> computed using [H,x]=p\hbar/m. The exchange potential is nonlocal,  
>> so its commutator with x will yield an additional term that is not  
>> accounted for. Not sure how important it is in practice. Do I  
>> remember correctly that epsilon.x also does not take into account  
>> the nonlocal pseudopotential contribution?
>>
>> Paolo
>>
>> On Wed, Apr 8, 2020 at 4:29 PM Manu Hegde <mhegde at sfu.ca  
>> <mailto:mhegde at sfu.ca>> wrote:
>>
>>    Hi Michal,
>>    Yes, it is possible.I did use both supercell and hybrid
>>    calculations. It did work.
>>    Manu
>>
>>    On Wed, Apr 8, 2020 at 10:08 AM Michal Krompiec
>>    <michal.krompiec at gmail.com <mailto:michal.krompiec at gmail.com>> wrote:
>>
>>        Hello,
>>        Is it possible to use epsilon.x on results of a calculation with a
>>        hybrid functional (supercell, gamma point only)?
>>
>>        Thanks,
>>
>>        Michal Krompiec
>>        Merck KGaA
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>>
>> -- 
>> Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
>> Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
>> Phone +39-0432-558216, fax +39-0432-558222
>>
>>
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>
> -- 
> Lorenzo Paulatto - Paris
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GIUSEPPE MATTIOLI
CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
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E-mail: <giuseppe.mattioli at ism.cnr.it>