[Pw_forum] highest occupied state

[Lorenzo Paulatto]

On Tuesday, 26 July 2016 20:15:34 CEST ZARA NBSH wrote:
> Dear users,
> I would like to calculate the highest occupied state energy of a
> nano-structure with respect to the vacuum level.
> 
> I took vacuum about 20A, if I increase the vacuum the total energy and gap
> do not change but the highest occupied and the lowest occupied states will
> be shifted up with a same amount.
> I can't understand the meaning of this energy shift,
> how can I calculate these energy respect to the vacuum level?


Absolute energies have no meaning in periodic boundary condition, because 
there is no "outside", for the same reason the potential is usually defined to 
have zero average value, as the convention of setting it to zero at infinity 
does not make sense.

For this reason, taking a very large cell won't help you much, as the level of 
the potential in the vacuum will only go to zero as the occupied fraction of 
the unit cell volume goes to zero, e.g. for a cell of 25A, of which 5A are 
occupied by ions, and 20A are vacuum, you'll still be off by at least 20%.

What you have to do, is measure the average value of the potential in vacuum, 
far away from the ions, and refer the energy levels to that. This procedure is 
done in the Work-Function example, that yo ucan find in PP/examples/
WorkFct_example. 

HTH, Lorenzo

-- 
Dr. Lorenzo Paulatto
IdR @ IMPMC -- CNRS & Université Paris 6
+33 (0)1 44 275 084 / skype: paulatz
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