[QE-users] How does one calculate dipole moments or access (readable) wave functions from e.g. PW?

[Thomas Brumme]

Dear Brian,

Sorry to be a bit blunt but your question cannot be answered in a 
one-liner and indicates that
you might miss a few important points.

I guess you have read the input description of bands.x and since you're 
are still asking how to
get the dipole matrix elements you are probably unaware of the fact that 
those are not well
defined in solids. Well, at least the position operator is not well 
defined in an infinite solid
because every point could be the origin. Thus, one has to use the 
momentum operator instead.
(see, e.g., Eq. (7) in this paper 
https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.80.1800)

I suggest to read some publications first, books, in order to understand 
what you need to do.
This paper seems quite useful - I didn't read it carefully though but 
just found it via a quick
Google search:

https://pubs.rsc.org/en/content/articlelanding/2023/CP/D2CP05991F

If you're interested in optical properties then there are many more 
things to take into account.
A good starting point might be the pages of the Yambo code, in 
particular this:

https://www.yambo-code.eu/wiki/index.php/Main_Page

and more specifically the thesis of Andrea Marini has a good collection 
of the equations and
references

https://www.yambo-code.eu/wiki/images/2/20/Marini_PhD.pdf

In a nutshell: you need to calculate the dielectric function and then 
there are different
approximations involved.

Kind regards

Thomas

On 8/9/23 09:27, Brian de Keijzer wrote:
> Hi all,
>
>
> Would anyone know how one retrieves the dipole matrix elements from bands.x (or in another way?)?
>
> Any help is appreciated.
>
>
> Bests,
>
> Brian de Keijzer
>
>> On 25 Jul 2023, at 16:09, Brian de Keijzer <briandekeyzer at hotmail.com> wrote:
>>
>> Hi Paolo,
>>
>> Thank you for your answer’
>>
>> Bands.x had caught my eye before but I did not notice how to make it output transition dipole moments. How would one compute those?
>>
>> Meanwhile I managed to indeed get hdf5 output. That’s great! If there is a native way to obtain the values I want it’s preferable tho.
>>
>> Best regards,
>>
>> Brian de Keijzer
>>
>>> On 25 Jul 2023, at 13:37, Paolo Giannozzi <paolo.giannozzi at uniud.it> wrote:
>>>
>>> I think code "bands.x" can compute dipole matrix elements.
>>>
>>> About direct access to wavefunctions: the default fortran binary format is basically unreadable except via the routines of QE. One may optionally write hdf5 files that are much easier to read with external utilities, though.
>>>
>>> Paolo
>>>
>>>> On 7/25/23 10:40, Brian de Keijzer wrote:
>>>> [You don't often get email from briandekeyzer at hotmail.com. Learn why this is important at https://aka.ms/LearnAboutSenderIdentification ]
>>>> Hi all,
>>>> New QE users here. I’ve used QE so far to compute band structures for crystals. I am however very much interested in calculating the dipole matrix elements for said crystals. Does QE have a module that allows one to do this? If so, how would that work?
>>>> Moreover, I have read that it is possible to use the wave functions directly. That would allow me to compute the dipole elements. Unfortunately most posts that I come across appear to be a bit dated tho. What would be the best way to access the wave functions using more recent versions of e.g. Python? I’ve come across postqe but that appears to no longer be in development nor does the installation work on my Python 3.9 distribution.
>>>> Anyhow; I’m looking to learn how to compute k-dependent transition dipole moments in QE. It would be awesome if anyone would want to provide an outline of how such a thing is to be done using the latest version of QE.
>>>> Bests,
>>>> Brian de Keijzer
>>>> _______________________________________________
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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
> _______________________________________________
> The Quantum ESPRESSO community stands by the Ukrainian
> people and expresses its concerns about the devastating
> effects that the Russian military offensive has on their
> country and on the free and peaceful scientific, cultural,
> and economic cooperation amongst peoples
> _______________________________________________
> Quantum ESPRESSO is supported by MaX (www.max-centre.eu)
> users mailing list users at lists.quantum-espresso.org
> https://lists.quantum-espresso.org/mailman/listinfo/users

-- 
Dr. rer. nat. Thomas Brumme
Theoretical chemistry
TU Dresden - KOE / 103
Bergstr. 66c
01069 Dresden

Tel:  +49 (0)351 463 39449

email: thomas.brumme at tu-dresden.de