[CP2K-user] [CP2K:22300] Re: On the conceptual meaning of magnetic related directives

[Elise Kenny]

Hello,

I'm just wanting to further clarify how to enter magnetization. 

The manual states MAGNETIZATION: "*The magnetization used in the atomic 
initial guess. Adds magnetization/2 spin-alpha electrons and removes 
magnetization/2 spin-beta electrons*"

However, according to the response above, should it say "*Adds 
magnetization*2 spin-alpha electrons and removes magnetization*2 spin-beta 
electrons"?*
So that magnetization=(number_alpha-number_beta)/2

Just want to make sure I'm entering it correctly.

Thanks in advance!

Elise
On Wednesday, 26 April 2023 at 8:01:33 pm UTC+10 Sigma-Jun Lu wrote:

> 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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