[QE-users] Spin counting in hp.x with paramagnetic metals

[Iurii Timrov]

Dear Theo,


Thanks for clarifications!


> I am typically looking at pressure-induced structural transitions in systems with complex magnetic ground states. However, in some cases the exact magnetic configurations are unknown and so to simplify the problem (and since these structural transitions occur at high temperature in the paramagnetic state) I have been trying to perform the calculations in the nonmagnetic state. It is in these instances where I see this approximate doubling of U: i.e. when I allow the system to relax into a FM ordered state (with nspin=2) before using hp.x the value of U ends up being around half that of when I don’t include spin effects (nspin=1).


I have not seen anything like that. You can have a closer look in to the HP code and report if you find any bugs. But I would need to understand better your system in order to figure out what is happening. In general terms it is hard to say anything more specific.


Regards,

Iurii


--
Dr. Iurii TIMROV
Senior Research Scientist
Theory and Simulation of Materials (THEOS)
Swiss Federal Institute of Technology Lausanne (EPFL)
CH-1015 Lausanne, Switzerland
+41 21 69 34 881
http://people.epfl.ch/265334
________________________________
From: Theo Weinberger <tiw21 at cam.ac.uk>
Sent: Tuesday, September 12, 2023 11:41:04 AM
To: Iurii Timrov; Quantum ESPRESSO users Forum
Subject: RE: Spin counting in hp.x with paramagnetic metals

Dear Iurii,

Yes, to be more precise I mean that when I am looking at these systems, I am modelling them as being nonmagnetic and so I was wondering whether it was the spin degeneracy that was causing this difference.

I am typically looking at pressure-induced structural transitions in systems with complex magnetic ground states. However, in some cases the exact magnetic configurations are unknown and so to simplify the problem (and since these structural transitions occur at high temperature in the paramagnetic state) I have been trying to perform the calculations in the nonmagnetic state. It is in these instances where I see this approximate doubling of U: i.e. when I allow the system to relax into a FM ordered state (with nspin=2) before using hp.x the value of U ends up being around half that of when I don’t include spin effects (nspin=1).

I hope this clarifies what I have been seeing.

Best wishes,

Theo Weinberger
PhD Student
University of Cambridge

From: users <users-bounces at lists.quantum-espresso.org> On Behalf Of Iurii Timrov via users
Sent: Tuesday, September 12, 2023 10:31 AM
To: users at lists.quantum-espresso.org
Subject: Re: [QE-users] Spin counting in hp.x with paramagnetic metals


Dear Theo,



How do you model the paramagnetic state? Please note that "paramagnetic" and "nonmagnetic" state is not the same thing.



For nonmagnetic calculations there is a factor of 2 due to spin degeneracy when computing sums over electronic states (i.e. in charge density, occupation matrix, and other quantities).  If you take a nonmagnetic material (e.g. LiCoO2) and model it as a spin-polarized system (nspin=2), the value of U will be the same as when modeling it as nonmagetic (because the magnetization will be zero). So I do not understand why do you have a factor of 2 difference for U in your simulations.



Greetings,

Iurii


--
Dr. Iurii TIMROV
Senior Research Scientist
Theory and Simulation of Materials (THEOS)
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 Theo Weinberger <tiw21 at cam.ac.uk<mailto:tiw21 at cam.ac.uk>>
Sent: Tuesday, September 12, 2023 11:21:03 AM
To: users at lists.quantum-espresso.org<mailto:users at lists.quantum-espresso.org>
Subject: [QE-users] Spin counting in hp.x with paramagnetic metals

Dear Quantum Espresso Users,

I have been using the hp.x code to calculate the Hubbard-U corrections for correlated metallic systems in both their spin-polarised and paramagnetic states.

In several materials I have noticed that the Hubbard-U value determined for a material in its paramagnetic ground state is approximately twice that compared to when a spin-polarised ground state is assumed (with all other parameters kept the same).

I was wondering whether anyone had any insight into how the accounting for spins in occupied Hubbard states works for the hp.x code and whether the paramagnetic implementation of hp.x perhaps counts all spins states twice resulting in this larger value.

Thank you in advance,

Theo Weinberger
PhD Student
University of Cambridge
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