Many DFT paper comare DOS by fermi energy set to zero, and some of them tell band shift by DOS.
As i know, fermi energy is highest occupied energy level at 0K.
And fermi level is equal to fermi energy at 0K.
But when we doped some material, fermi level can be changed. in this case, What is the meaning that comparing with DOS by fermi energy set to zero?
I think that comparing with fermi energy just only can tell energy gap from fermi energy to conduction band and valence band, not band shift (upward or downward)
Is it comparing by vacuum level better to explain band shift?
Why people compare DOS refer to fermi energy?
In the most general terms, absolute energy is meaningless; only energy differences are observables.
Choosing the fermi level as our "0" is very convenient when studying a single material because it is one of the things we can easily calculate.
Vacuum energy is also a valid choice of zero, and as you say is more appropriate for comapring band alignment between different materials.
The reason vacuum energy is not calculated "as standard" is often due to the way we have designed most DFT codes - modelling a bulk solid in periodic boundary conditions has *no vacuum* and therefore an additional slab calculation (i.e. a material-vacuum interface) needs to be calculated to find E_Fermi relative to the vacuum energy.
In the plane-wave codes that many people use, vacuum costs as much as matter in terms of minimisation, making these calculations *not cheap*.
In short; yes, you are right, but if you aren't comparing different materials in your project, vacuum levels are a lot of extra work for no benefit!
Hope that helps!
To Robert Lawrence
I really appreciate your answer.
But I still confuse meaning of refer to fermi level.
For example, If I want to compare 3 material TiO2, p-type doping TiO2, n-type doping TiO2 ,
then p doping will shift fermi level to Valence band
and n doping will shift fermi level to conduction band
Comparing DOS of 3 material by fermi level set to zero, I think the information from these DOS is just showing fermi level shift from VB and CB easier. Although the 3 material is same material TiO2
Still I feel that comparing by fermi level need less resource but show few informations.
What i missed?
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