[CP2K-user] [CP2K:18725] Re: On the conceptual meaning of magnetic related directives

[Sigma-Jun Lu]

As far as i can concern:
1)Regularly, if alpha/beta corresponds to spin polarizability, there should 
be a relationshipf between charge/spin(specifically for each orbital, or 
combination of them as a whole, the quantity to set total spin is 
magnetization): 
charge=-(alpha+beta)/2, magnetization=(alpha-beta)/2, or inversely 
alpha=magnetization-charge, beta=-charge-magnetization.

2) Multiplicity, is a little bit complex concept. Normally, you can only 
use 0~2 in cp2k--
0: not consider the spin contribution;
1: fixed FM state without flipping of spins;
2: consider two degree of freedom, up and down;
3 and above, only consider deeply magnetic interaction, rather than single 
electrons, take 2 electrons for example, you might get one singlet plus a 
set of triplets. * i have not seen any signal that cp2k can handle this 
issue.

3) Perhaps you need add more MOs for proper smearing, usually, empty 
orbitals does not dirty your calculation result of gap energy.

4) Use $BS instead with seperating atoms of spin up and down in your &coord 
section, and forget multiplity and magnetization with a # symbol or 
deleting the line. See page 35/36 in this document- 
https://www.cp2k.org/_media/events:2015_cecam_tutorial:ling_hybrids.pdf . 
Nevertheless, during the optimization process, you may not use $BS, but 
magnetization, and foget #multiplicity.

hope these helps.

Sigma-Jun LU

在2021年5月5日星期三 UTC+8 15:29:17<ivansc... at gmail.com> 写道：

> Dear all,
>
> I come across three different directives, which are all related but 
> confused me, somehow. According to the definition of the manual, we find
>
> MULTIPLICITY: "*Two times the total spin plus one. Specify 3 for a 
> triplet, 4 for a quartet, and so on. Default is 1 (singlet) for an even 
> number and 2 (doublet) for an odd number of electrons.*" 
>
> FIXED_MAGNETIC_MOMENT: "*Imposed difference between the numbers of 
> electrons of spin up and spin down: m = n(up) - n(down). A negative value 
> (default) allows for a change of the magnetic moment. -1 specifically keeps 
> an integer number of spin up and spin down electrons.*"
>
>  MAGNETIZATION: "*The magnetization used in the atomic initial guess. 
> Adds magnetization/2 spin-alpha electrons and removes magnetization/2 
> spin-beta electrons*"
>
> I am trying to set magnetic simulations with and without DFT+U corrections 
> for bulk and slabs, both for Ferro-Magnetic (FM) and Anti-Ferro-Magnetic 
> (AFM) ordering. From my experience with other codes, it is first required 
> to set the initial magnetization for the atomic sites. Thus, I would use 
> directive *magnetization* (BS can be also used but we will not consider 
> it here) when setting the &SUBSYS block. However, I am not sure what this 
> magnetization/2 spin-alpha (-beta) means. 
> *Question 1) Can you clarify the meaning of the magnetization/2 is for 
> alpha and beta? What are the units for the magnetization? *
>
> Now let us consider two different cases.
> *Case A. Solution via diagonalization, smearing and mixing*
> In section &SMEAR we can set FIXED_MAGNETIC_MOMENT to define the 
> difference between electrons up-down (if positive) or allow it to change 
> (if negative). This is very convenient for FM ordering calculations. 
> However, I find it confusing what I should set for MULTIPLICITY in this 
> case. For AFM, one should set FIXED_MAGNETIC_MOMENT=0
> and MULTIPLICITY=1. 
>
> *Question 2) For FM systems, one can set the value of 
> FIXED_MAGNETIC_MOMENT and the corresponding value for MULTIPLICITY. 
> However, if I set FIXED_MAGNETIC_MOMENT < 0 (to allow the magnetization to 
> change) what should be the value for MULTIPLICITY? Should it be 0.0? What 
> is the meaning of defining MULTIPLICITY = 0.0 (default)?*
>
> *Case B. Solution only via OT for systems with band gap*
> In this case there is no need to introduce &SMEAR. 
> *Question 3: CP2K still runs if one sets &OT+&SMEAR+(ADDED_MOL>0, why? 
> Should not the code prevent such simulation?*
> Assuming that we do not include &SMEAR, thus we do not have a way to 
> define the value for FIXED_MAGNETIC_MOMENT. Even tough we can set the 
> initial magnetization, the only way to control the total magnetization/spin 
> is through MULTIPLICITY, but I am not sure if this works correctly for 
> condensed phase systems.
> *Question 4: For AFM states, could I set an initial value of zero from the 
> initial MAGNETIZATION values, and set MULTIPLICITY=1? How do I properly FM 
> states with OT using MAGNETIZATION and MULTIPLICITY?*
>
> Thanks a lot for your patience and help to answer my questions
> Ivan
>
>
>

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