[QE-users] Momentum matrix element on PBE+U

Fri May 15 21:02:09 CEST 2020

> I will now need to look at the implementation of this part, but the implementation is quite complex with almost no comments. With your experience, do you have any idea or hunch why there is a problem in the calculation at the Gamma point?

I have no idea why you have these issues. You can have a look in the routine commutator_Hx_psi.f90 which does the same thing. So maybe this can help you.

> Can you refer me to references or a person that possibly can help?

I think that part of the code was written by Prof. Andrea Dal Corso.

Greetings,

Iurii

--
Dr. Iurii Timrov
Postdoctoral Researcher
STI - IMX - THEOS and NCCR - MARVEL
Swiss Federal Institute of Technology Lausanne (EPFL)
CH-1015 Lausanne, Switzerland
+41 21 69 34 881
http://people.epfl.ch/265334
________________________________
From: users <users-bounces at lists.quantum-espresso.org> on behalf of L Y <xifntx at gmail.com>
Sent: Friday, May 15, 2020 7:12:45 PM
To: Quantum ESPRESSO users Forum
Subject: Re: [QE-users] Momentum matrix element on PBE+U

Wow thanks Lurii for all these very helpful references and comments! I read the section in your thesis and looked at the references. By commenting out the part of the [r, V_nl] calculation in "compute_ppsi", I think I have narrowed down the issue to the calculation of this commutator. Attached are figures of velocity_{cv} with and without this commutator, along symmetry lines. Near the Gamma point (ik=31), a dip is seen in the figure with nonlocal part (in other calculations with USPP and PAW this discontinuity) is much more pronounced.
I will now need to look at the implementation of this part, but the implementation is quite complex with almost no comments. With your experience, do you have any idea or hunch why there is a problem in the calculation at the Gamma point? Can you refer me to references or a person that possibly can help?

Best regards,
Lun Yue
Louisiana State University

It is now clear to me that "bug" in QE is in the part

Den fre. 15. maj 2020 kl. 04.13 skrev Timrov Iurii <iurii.timrov at epfl.ch<mailto:iurii.timrov at epfl.ch>>:

> Did you mean [J. Chem. Phys. 120, 9935 (2004)] by Toby and Dal Corso?

The correct reference (without mistakes) is this: J. Tóbik<https://aip.scitation.org/author/T%C3%B3bik%2C+Jaroslav> and A. Dal Corso<https://aip.scitation.org/author/Dal+Corso%2C+Andrea>, J. Chem. Phys. 120, 9934 (2004).

See Eq. (7) in this paper, which is for the ultra-soft pseudos. For norm-concerning pseudos, see Eq. (14) in S. Baroni and R. Resta, Phys. Rev. B 33, 7017 (1986).

> I could not find an expression for the momentum matrix element in that reference, I think that reference is only about the extra term involving the ultrasoft pseudopotential?

Let me try to explain for the corn-conserving case, which is simpler. The routine "compute_ppsi" has p - i [x, V_{nl}], where p is the momentum and V_{nl} is the nonlocal part of the pseudopotential. See the right-hand side of Eq. (15) in Phys. Rev. B 33, 7017 (1986). See also Eqs. (8) and (9) in

A. Marini, G. Onida, and R. Del Sole, Phys. Rev. B 64, 195125 (2001).

So actually the code computes not only the matrix element of the momentum p, but also it contains an extra contribution due to the non-local part of the pseudopotential. In the DFT+U case, there is yet another term given by Eq. (53) in Phys. Rev. B 101, 064305 (2020).

Finally, some related discussion about this (how to compute an electron velocity using these "tricks") is given in Sec. 6.1.3(a) of my PhD thesis: https://pastel.archives-ouvertes.fr/pastel-00823758/

HTH

Iurii

--
Dr. Iurii Timrov
Postdoctoral Researcher
STI - IMX - THEOS and NCCR - MARVEL
Swiss Federal Institute of Technology Lausanne (EPFL)
CH-1015 Lausanne, Switzerland
+41 21 69 34 881
http://people.epfl.ch/265334
________________________________
From: users <users-bounces at lists.quantum-espresso.org<mailto:users-bounces at lists.quantum-espresso.org>> on behalf of L Y <xifntx at gmail.com<mailto:xifntx at gmail.com>>
Sent: Thursday, May 14, 2020 9:15:47 PM
To: Quantum ESPRESSO users Forum
Subject: Re: [QE-users] Momentum matrix element on PBE+U

Dear Lurii,

Thank you for the quick response!
Did you mean [J. Chem. Phys. 120, 9935 (2004)] by Toby and Dal Corso? I could not find an expression for the momentum matrix element in that reference, I think that reference is only about the extra term involving the ultrasoft pseudopotential?
I ask because I have calculated the momentum matrix elements between valence and conduction bands along symmetry lines in MgO. Everything seems OK, except the fact that there is a noticeable discontinuity of the matrix element at the Gamma point. I did extensive debugging, but could find anything wrong (the band energies are fine and continuous without discontinuity). I suspect I need to do something special for the momentum matrix element, but I cannot figure out the expression.

Best regards,
Lun Yue
Louisiana State University

Den tor. 14. maj 2020 kl. 11.55 skrev Timrov Iurii <iurii.timrov at epfl.ch<mailto:iurii.timrov at epfl.ch>>:

Dear Lun Yue,

The reference is written in the header of that routine.

Greetings,

Iurii

--
Dr. Iurii Timrov
Postdoctoral Researcher
STI - IMX - THEOS and NCCR - MARVEL
Swiss Federal Institute of Technology Lausanne (EPFL)
CH-1015 Lausanne, Switzerland
+41 21 69 34 881
http://people.epfl.ch/265334
________________________________
From: users <users-bounces at lists.quantum-espresso.org<mailto:users-bounces at lists.quantum-espresso.org>> on behalf of lot fun <xifntx at gmail.com<mailto:xifntx at gmail.com>>
Sent: Thursday, May 14, 2020 6:45:16 PM
To: Quantum ESPRESSO users Forum
Subject: Re: [QE-users] Momentum matrix element on PBE+U

Dear Lurii,

I was wondering whether you have any reference on how the momentum matrix element is implemented in QE?
Obviously the PP/src/compute_ppsi.f90 routine is crucial, but I dunno which formula is used there. A reference to a paper would be greatly appreciated!

Best regards,
Lun Yue
Louisiana State University

Den ons. 13. maj 2020 kl. 03.06 skrev Timrov Iurii <iurii.timrov at epfl.ch<mailto:iurii.timrov at epfl.ch>>:

> I encounter this error: 'Error in routine write_p_avg (1): write_p_avg not working with LDA+U'.

The LDA+U does not work in this case because there is a missing term, see Eq. (53) in PRB 101, 064305 (2020).

> Is there a way to circumvent this on Quantum Espresso?

I do not know any apart from adding the missing term.

Iurii

--
Dr. Iurii Timrov
Postdoctoral Researcher
STI - IMX - THEOS and NCCR - MARVEL
Swiss Federal Institute of Technology Lausanne (EPFL)
CH-1015 Lausanne, Switzerland
+41 21 69 34 881
http://people.epfl.ch/265334
________________________________
From: users <users-bounces at lists.quantum-espresso.org<mailto:users-bounces at lists.quantum-espresso.org>> on behalf of Shivesh Sivakumar <shiveshsivakumar at gmail.com<mailto:shiveshsivakumar at gmail.com>>
Sent: Wednesday, May 13, 2020 8:14:25 AM
To: Quantum ESPRESSO users Forum
Subject: [QE-users] Momentum matrix element on PBE+U

Hello users,

Sorry for the repost, but I think my question went unnoticed.
Is there a way to calculate the momentum matrix element in PBE+U on QE? I encounter this error: 'Error in routine write_p_avg (1): write_p_avg not working with LDA+U'. Is there a way to circumvent this on Quantum Espresso?

Best,
Shivesh Sivakumar
University of Washington
Seattle-WA, 98105

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