The understanding of `occ` variable

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skyair
Posts: 1
Joined: Sun Aug 11, 2019 11:03 am

The understanding of `occ` variable

Post by skyair » Sun Aug 11, 2019 11:19 am

I cannot understand the ussage of occ variable.

The all variables help file mentioned
Typical band occupancy is either 2 or 0, but can be 1 for half-occupied band or other choices in special circumstances.


Thus, I think that It should be 0 if the band is un-occupied, 1 if the band is half-occupied and 2 if the band is full-occupied. But I saw an input file (https://docs.abinit.org/tests/v3/Input/t19.in) which broke my understanding by 6*1/3. I want to know what one thrid means. Is there are some troubles on my thought?

In a nutshell, could you tell me what OCCupation numbers especially one third means on earth?

gonze
Posts: 412
Joined: Fri Aug 14, 2009 8:29 pm

Re: The understanding of `occ` variable

Post by gonze » Wed Aug 21, 2019 2:51 pm

Dear skyair,
Although we know that electrons cannot be split, there is no problem mathematically to have fractional occupancies in density functional theory.
Please, have a look at :
- JP Perdew et al, Phys. Rev. Lett 49, 1691 (1982)
- Gross et al Phys Rev A 37, 2809 (1988)
There are many others references about fractional occupations ...
Best

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