Discussion:
[Pw_forum] Zero of the electrostatic potential ?
Vladan Stevanovic
2009-06-23 09:29:28 UTC
Permalink
Dear Quantum-ESPRESSO users,

I am using QE for simulations of transition metal clusters. My question
is where the code places the vacuum level in such calculations
(nonperiodic systems in a big cell) ? To formulate differently, is it
possible to compare directly KS eigenvalues from different calculations ?


Thanks,
Vladan Stevanovic

---------------------------------------------
Assistant-doctorant
EPFL SB IRRMA-GE
PH H2 435 (B?timent PH)
Station 3
CH-1015 Lausanne
tel. : +41 (0)21 693 51 08
e-mail : vladan.stevanovic at epfl.ch
---------------------------------------------
Gabriele Sclauzero
2009-06-23 09:46:13 UTC
Permalink
Post by Vladan Stevanovic
Dear Quantum-ESPRESSO users,
I am using QE for simulations of transition metal clusters. My question
is where the code places the vacuum level in such calculations
(nonperiodic systems in a big cell) ? To formulate differently, is it
I think this question has been often posed and replied in the context of work-function
calculation. Please first try a search on the archives using 'work function' or something
similar as keys. You can also have a look to the example in

examples/WorkFct_example

in the QE folder.
Post by Vladan Stevanovic
possible to compare directly KS eigenvalues from different calculations ?
If you want compare the distance between the eigenvalues and the Fermi energies in the
respective systems, I think you can. In order to understand if you can compare the
absolute (i.e. not relative to E_F) value of the eigenvalues I think you must first find
the answer to your previous question.


GS
Post by Vladan Stevanovic
Thanks,
Vladan Stevanovic
---------------------------------------------
Assistant-doctorant
EPFL SB IRRMA-GE
PH H2 435 (B?timent PH)
Station 3
CH-1015 Lausanne
tel. : +41 (0)21 693 51 08
e-mail : vladan.stevanovic at epfl.ch
---------------------------------------------
_______________________________________________
Pw_forum mailing list
Pw_forum at pwscf.org
http://www.democritos.it/mailman/listinfo/pw_forum
--
o ------------------------------------------------ o
| Gabriele Sclauzero, PhD Student |
| c/o: SISSA & CNR-INFM Democritos, |
| via Beirut 2-4, 34014 Trieste (Italy) |
| email: sclauzer at sissa.it |
| phone: +39 040 3787 511 |
| skype: gurlonotturno |
o ------------------------------------------------ o
Lorenzo Paulatto
2009-06-23 10:00:01 UTC
Permalink
In data 23 giugno 2009 alle ore 11:29:28, Vladan Stevanovic
Post by Vladan Stevanovic
I am using QE for simulations of transition metal clusters. My question
is where the code places the vacuum level in such calculations
(nonperiodic systems in a big cell) ?
Dear Vladan,
a non-periodic system in a big periodic cell is still a periodic system!
Hence, the integral of the potential over any possible choice of primitive
cell has to be zero. This is a sufficient condition to find the zero.
Post by Vladan Stevanovic
To formulate differently, is it
possible to compare directly KS eigenvalues from different calculations ?
In general it is NOT possible to directly compare eigenvalues of different
calculations, nevertheless they can be compared taking the Fermi energy as
reference.

best regards
--
Lorenzo Paulatto
SISSA & DEMOCRITOS (Trieste)
phone: +39 040 3787 511
skype: paulatz
www: http://people.sissa.it/~paulatto/

*** save italian brains ***
http://saveitalianbrains.wordpress.com/
Vladan Stevanovic
2009-06-23 13:17:10 UTC
Permalink
Thank you very much.

Vladan Stevanovic
---------------------------------------------
Assistant-doctorant
EPFL SB IRRMA-GE
PH H2 435 (B?timent PH)
Station 3
CH-1015 Lausanne
tel. : +41 (0)21 693 51 08
e-mail : vladan.stevanovic at epfl.ch
---------------------------------------------
Vladan Stevanovic
2009-06-23 13:17:10 UTC
Permalink
Thank you very much.

Vladan Stevanovic
---------------------------------------------
Assistant-doctorant
EPFL SB IRRMA-GE
PH H2 435 (B?timent PH)
Station 3
CH-1015 Lausanne
tel. : +41 (0)21 693 51 08
e-mail : vladan.stevanovic at epfl.ch
---------------------------------------------

Stefano de Gironcoli
2009-06-23 10:08:03 UTC
Permalink
The zero of the potential energy is the average electrostatic potential;
as such the eigenvalues in different systems cannot be directly compared.
what you need yo do is to calculate the (electrostatic) potential energy
in the vacuum region where it should, at least for neutral systems, be
constant and take that as the zero of the energy.
In order to calculate the potential in the vacuum region one could adapt
the example for the calculation of the Work Function in directory
examples/WorkFct_example.

stefano
Post by Vladan Stevanovic
Dear Quantum-ESPRESSO users,
I am using QE for simulations of transition metal clusters. My question
is where the code places the vacuum level in such calculations
(nonperiodic systems in a big cell) ? To formulate differently, is it
possible to compare directly KS eigenvalues from different calculations ?
Thanks,
Vladan Stevanovic
---------------------------------------------
Assistant-doctorant
EPFL SB IRRMA-GE
PH H2 435 (B?timent PH)
Station 3
CH-1015 Lausanne
tel. : +41 (0)21 693 51 08
e-mail : vladan.stevanovic at epfl.ch
---------------------------------------------
_______________________________________________
Pw_forum mailing list
Pw_forum at pwscf.org
http://www.democritos.it/mailman/listinfo/pw_forum
Paolo Giannozzi
2009-06-23 10:07:10 UTC
Permalink
Post by Vladan Stevanovic
where the code places the vacuum level in such calculations
(nonperiodic systems in a big cell) ? To formulate differently,
is it possible to compare directly KS eigenvalues from different
calculations ?
short answer: no.

V(G=0) is calculated (finite or infinite systems doesn't matter)
as described in the attached notes. CP and PWscf have a
different definition of V(0) so you get the same eigenvalues,
but displaced

P.
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---
Paolo Giannozzi, Dept of Physics, University of Udine
via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
Vladan Stevanovic
2009-06-23 09:29:28 UTC
Permalink
Dear Quantum-ESPRESSO users,

I am using QE for simulations of transition metal clusters. My question
is where the code places the vacuum level in such calculations
(nonperiodic systems in a big cell) ? To formulate differently, is it
possible to compare directly KS eigenvalues from different calculations ?


Thanks,
Vladan Stevanovic

---------------------------------------------
Assistant-doctorant
EPFL SB IRRMA-GE
PH H2 435 (B?timent PH)
Station 3
CH-1015 Lausanne
tel. : +41 (0)21 693 51 08
e-mail : vladan.stevanovic at epfl.ch
---------------------------------------------
Gabriele Sclauzero
2009-06-23 09:46:13 UTC
Permalink
Post by Vladan Stevanovic
Dear Quantum-ESPRESSO users,
I am using QE for simulations of transition metal clusters. My question
is where the code places the vacuum level in such calculations
(nonperiodic systems in a big cell) ? To formulate differently, is it
I think this question has been often posed and replied in the context of work-function
calculation. Please first try a search on the archives using 'work function' or something
similar as keys. You can also have a look to the example in

examples/WorkFct_example

in the QE folder.
Post by Vladan Stevanovic
possible to compare directly KS eigenvalues from different calculations ?
If you want compare the distance between the eigenvalues and the Fermi energies in the
respective systems, I think you can. In order to understand if you can compare the
absolute (i.e. not relative to E_F) value of the eigenvalues I think you must first find
the answer to your previous question.


GS
Post by Vladan Stevanovic
Thanks,
Vladan Stevanovic
---------------------------------------------
Assistant-doctorant
EPFL SB IRRMA-GE
PH H2 435 (B?timent PH)
Station 3
CH-1015 Lausanne
tel. : +41 (0)21 693 51 08
e-mail : vladan.stevanovic at epfl.ch
---------------------------------------------
_______________________________________________
Pw_forum mailing list
Pw_forum at pwscf.org
http://www.democritos.it/mailman/listinfo/pw_forum
--
o ------------------------------------------------ o
| Gabriele Sclauzero, PhD Student |
| c/o: SISSA & CNR-INFM Democritos, |
| via Beirut 2-4, 34014 Trieste (Italy) |
| email: sclauzer at sissa.it |
| phone: +39 040 3787 511 |
| skype: gurlonotturno |
o ------------------------------------------------ o
Lorenzo Paulatto
2009-06-23 10:00:01 UTC
Permalink
In data 23 giugno 2009 alle ore 11:29:28, Vladan Stevanovic
Post by Vladan Stevanovic
I am using QE for simulations of transition metal clusters. My question
is where the code places the vacuum level in such calculations
(nonperiodic systems in a big cell) ?
Dear Vladan,
a non-periodic system in a big periodic cell is still a periodic system!
Hence, the integral of the potential over any possible choice of primitive
cell has to be zero. This is a sufficient condition to find the zero.
Post by Vladan Stevanovic
To formulate differently, is it
possible to compare directly KS eigenvalues from different calculations ?
In general it is NOT possible to directly compare eigenvalues of different
calculations, nevertheless they can be compared taking the Fermi energy as
reference.

best regards
--
Lorenzo Paulatto
SISSA & DEMOCRITOS (Trieste)
phone: +39 040 3787 511
skype: paulatz
www: http://people.sissa.it/~paulatto/

*** save italian brains ***
http://saveitalianbrains.wordpress.com/
Stefano de Gironcoli
2009-06-23 10:08:03 UTC
Permalink
The zero of the potential energy is the average electrostatic potential;
as such the eigenvalues in different systems cannot be directly compared.
what you need yo do is to calculate the (electrostatic) potential energy
in the vacuum region where it should, at least for neutral systems, be
constant and take that as the zero of the energy.
In order to calculate the potential in the vacuum region one could adapt
the example for the calculation of the Work Function in directory
examples/WorkFct_example.

stefano
Post by Vladan Stevanovic
Dear Quantum-ESPRESSO users,
I am using QE for simulations of transition metal clusters. My question
is where the code places the vacuum level in such calculations
(nonperiodic systems in a big cell) ? To formulate differently, is it
possible to compare directly KS eigenvalues from different calculations ?
Thanks,
Vladan Stevanovic
---------------------------------------------
Assistant-doctorant
EPFL SB IRRMA-GE
PH H2 435 (B?timent PH)
Station 3
CH-1015 Lausanne
tel. : +41 (0)21 693 51 08
e-mail : vladan.stevanovic at epfl.ch
---------------------------------------------
_______________________________________________
Pw_forum mailing list
Pw_forum at pwscf.org
http://www.democritos.it/mailman/listinfo/pw_forum
Paolo Giannozzi
2009-06-23 10:07:10 UTC
Permalink
Post by Vladan Stevanovic
where the code places the vacuum level in such calculations
(nonperiodic systems in a big cell) ? To formulate differently,
is it possible to compare directly KS eigenvalues from different
calculations ?
short answer: no.

V(G=0) is calculated (finite or infinite systems doesn't matter)
as described in the attached notes. CP and PWscf have a
different definition of V(0) so you get the same eigenvalues,
but displaced

P.
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---
Paolo Giannozzi, Dept of Physics, University of Udine
via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
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