Dear Shivesh Sivakumar
There is no difference in the calculations you mention, you can use
pp.x and average.x after HSE calculations in the case of 2D to 0D
systems (otherwise, in the case of fully periodic 3D systems it is
nonsense to use average.x to find the vacuum level as potential
asymptote...). However, the "physical relevance" of such calculations
is another matter: if you calculate the HOMO-vacuum level energy
difference, you are totally neglecting the rearrangement of electrons
in the N-1 (i.e., ionized) final state. Your crude estimate from
ground-state eigenvalues might be quite inaccurate. There are tons of
literature on this subject, and if you want to know more, you might
start from here: Dabo et al., Phys. Rev. B 2010, 82, 115121. In the
case of isolated systems (e.g., molecules or clusters), you can use
some different program, implementing localized orbital basis sets, and
calculate total energy differences such as E(N-1)-E(N), where N is the
number of electrons of the neutral species. In periodic/plane-wave
calculations this is not possible due to the fact that in charged
system a diffuse countercharge, necessary to the convergence of the
Ewald's sum, is added to the system, shifting the total energy. AFAIK,
you can simulate real ionization processes and calculate the
corresponding energies by using excited-state codes such as YAMBO,
that use the QE ground state to calculate several spectroscopic
observables.
HTH
Giuseppe
Quoting Shivesh Sivakumar <[email protected]>:
Hello users,
I was wondering whether the ionization energy calculated from a HSE
calculation has any physical relevance. I understand the mixing of exact
exchange gives us a more reliable estimate of the band gap, but is it
possible to comment on the ionization energy? I intend to do this the same
way as PBE, LDA etc - Calculate the vacuum level and then compute I.E. as
Vacuum - HOMO. I also want to know if pp.x and average.x are compatible
with hybrid functional calculations on QE. I've already calculated the IE
using PBE for my material but don't know what to expect for HSE.
Any comments or insights would be very appreciated.
Best,
Shivesh Sivakumar
University of Washington-Seattle
WA-98105
GIUSEPPE MATTIOLI
CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
Via Salaria Km 29,300 - C.P. 10
I-00015 - Monterotondo Scalo (RM)
Mob (*preferred*) +39 373 7305625
Tel + 39 06 90672342 - Fax +39 06 90672316
E-mail: <[email protected]>
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