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

[iva...@gmail.com]

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


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.cp2k.org/archives/cp2k-user/attachments/20210505/51b683d6/attachment.htm>