[Pw_forum] Core-Level Shifts and Charged Supercells Related Questions

[Evren Ataman]

Dear Quantum Espresso Users, Developers and Everybody in Between

(Sorry for the loong e-mail, which may take time equivalent of drinking a
cup of espresso, to read)
(I have tried to follow the "posting guidelines" in the Quantum Espresso
page before posting. I hope this e-mail is an appropriate one)
(To make the e-mail easy to read for time-lacking people I have put $$$ in
front of every question)
(If I solve my problems and understand everything, I promise to take the
responsibility to reply similar questions in the forum in 2014)

Thank you very much in advance.

INTRO:
I have a problem which results from my partial or complete lack of
understanding related to couple of points.
My ultimate purpose is to calculate surface core-level shifts for couple of
elements in an insulator (I prefer to keep the material confidential) to
compare/guide to XPS measurements.
I eventually want to use PAW pseudopotentials (PP) and generate my own
(with and without the core-hole on the appropriate energy level) by using
AtomPaw code. I have not used ld1.x because of two reasons. Firstly, with
my all respect to ld1.x, I personally found it a bit more complicated for a
beginner to begin with. Secondly, for AtomPaw code the input files for
well-tested PPs are very easily available so that one can just remove one
core-electron and good to go (I hoped).

THE CASE STUDY 1 (Benzene Molecule):
To understand the concept and warm myself up I first made some trivial
calculations with a benzene molecule using PPs generated by experts
(details are coming).
When I calculate the benzene molecule with ordinary PPs, in the output file
I get number of electrons as 30. This number makes sense: 6 carbon atoms
each with 4 valence electrons and 6 hydrogen atoms each with 1; add up to
30.
When I replace one of the carbon PP with a corresponding PP with 1s core
hole, in the output file I get 31 electrons.
The PP I use with the core-hole has the following info among others:

--------------------
<PP_INFO>
Generated using "atomic" code by A. Dal Corso  (espresso distribution)
Author: Paolo Giannozzi   Generation date:  7Feb2008
Info:   C PBE with a 1s core-hole: 1s1 2s2 2p2 RRKJ3 US
nl pn  l   occ               Rcut            Rcut US             E pseu
2S  1  0  2.00      1.40000000000      1.60000000000 -2.19814028365
2S  1  0  0.00      1.40000000000      1.60000000000 0.05000000000
2P  2  1  2.00      1.30000000000      1.60000000000 -1.56800948749
2P  2  1  0.00      1.30000000000      1.60000000000 0.05000000000
3D  3  2 -2.00      1.30000000000      1.30000000000 0.15000000000
<PP_HEADER>
5.00000000000      Z valence
Wavefunctions         nl  l   occ
                       2S  0  2.00
                       2P  1  2.00
--------------------

When I run this (an SCF calculation) I get the error message saying:
"charge is wrong: smearing is needed"
and a bit below a warning:
"WARNING: integrated charge=    30.30000000, expected= 31.00000000"
When I add smearing (occupations='smearing', degauss = 0.1,
smearing='gauss') fairly calculation runs without any problem but unfairly
WARNING disappears.

QUESTION SET NUMBER ONE:
$$$ Is this supercell charged with and/or without smearing?
    $$$ If yes how the charged-supercell is taken care of (is "a
compensating jellium background is inserted" automatically without
prompting the user)?
$$$ What is integrated charge and what is expected charge?
$$$ Why z_valance is 5?
    $$$ If one puts one of the 1s core electrons into the one of the
valence levels should not this make 2p level occupied by 3 electrons
instead of 2 and write this explicitly in the PP file as 1s2 2s2 2p3?
$$$ Does ld1.x calculate Z_valence as (atomic number) - (number of core
electrons) or it just counts the electrons in the valence level given by
the user?

ONE COMMENT AND TWO QUESTIONS:
I assume that a PP with a core electron moved to valence level can be quite
suitable for X-ray absorption process and with certain approximations for
XPS, too. But let say that I want to try the case with 4 valence electrons
and that somehow I create a PP for C atom with a core-hole and Z_valence
value of 4 and used in my trivial benzene calculation.
$$$ Would this supercell be charged?
    $$$ If yes how the charged-supercell is taken care of (is "a
compensating jellium background is inserted" automatically)?
$$$ In general, is it possible to generate a charged-supercell with Quantum
Espresso (or any other planeweve DFT code) just with ionized PPs or the
only way is to use appropriate keyword, total_charge?

CASE STUDY 2 (Generating PPs with AtomPaw) AND ONE QUESTION:
As far as I understood when one creates a PP with ld1.x one need to input
atomic symbol Xor atomic number. So inputting both "C" and "6" is not
allowed.
However in AtomPaw input, one has to write both C and 6 but in this case 6
represents the total number of electrons (this is what I understood from
the user manual).
The AtomPaw generates a lot of output files two of which are the actual PP
files: one with an .xml extension and one with an .upf extension for
Quantum Espresso.

When I generate a PP with a core-hole I assumed that total number of
electrons for C should be 5 with 1s1 2s2 2p2 configuration.
In this case in the .xml file I read: "<atom symbol= "C" Z="5" core="1"
valence="4"/>" and in the corresponding .upf file in the <PP_HEADER> I read
z_valence = 4.

When I generate a PP with a core-hole but this time with C and 6 electrons
(keeping everything else the same).
In the .xml file I read: "<atom symbol= "C" Z="6" core="1" valence="5"/>"
and in corresponding .upf file in the <PP_HEADER> I read z_valence = 4.
$$$ Why z_valence value is not correctly inserted into the .upf file, i.e.,
always becomes 4?


With my Kind regards

-- 

Evren Ataman
/University of Copenhagen/Department of Chemistry
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