[Pw_forum] qustions about the initio magnetization of ion

[Stefano de Gironcoli]

Dr. Shu-jun Hu wrote:
>
> Dear all,
>
> I have some qustions about the initio magnetization of ion for LSDA
> calculation.
>
> In the input file the "starting_magnetization" is used to break the
> spin symmetry of magnetic system. By specifying such parameters, how
> does pw.x initio the magnetic moment of certain ions?
> Taking transition metal ion Fe for example, if the
> starting_magnetizaton(Fe)=0.4, how many electrons occupy the spin-up
> and spin-down channels respectively for the initio wfc? Only the
> symmetry breaking of 3d electrons are considered in LSDA calculation,
> or all the valence electrons (both 3d and 4s)?
>
the starting magnetization variables define the way the initial initial
charge density, that is used to generate the initial potential, is
built. The initial density is obtained from the superposition of atomic
charges, read from the pseudopotential file, and in the case of a spin
polarized calculation the initial charge density is defined as
rho(r,sigma) = sum_s=1,Nat rho_at_type(r-Rs) * (1 + sigma*
starting_magnetization_type)/2
where sigma is +/- 1 for up/down spin components

the atomic charge density is the total one (including the 4s in case of Fe).

occupation of the initial wfs is NOT imposed and is the result of the
first diagomalization + smearing + fermi energy...

> Such a question concerns about the output information of calculation
> with constrained magnetization.
>
> During the calculation, I got the output information as:
>
> atom number 14 relative position : 1.4040 4.1784 3.2357
> charge : 7.082847
> magnetization : 0.239576
> magnetization/charge: 0.033825
> constrained moment : 0.200000
>
> Does the value of "magnetization" (0.239576) represent the magnetic
> moment of ion, or the polarization? If the latter case, how to derive
> the magnetic moment in unit of Bohr?
>
I may be wrong (I'm not usign this feature often) but inspecting the
code the meaning of the following output is:
there are 7.08... electrons in the chosen sphere around the atom # 14
there are 0.2395... more up electrons than down ones in the sphere...
this is more the local moment than the local magnetization
0.033 is the ratio of the two numbers above
0.20 is the target value of the magnetic moment (the actual value is as
said 0.2395...). the fact that the actual moment does not agree with the
target one is due to the fact that the constraint is impose via a
penalty function. the larger the value of lambda the closer should be
the computed value to its target.

> -------
>
> Another question is about the total energy given by constrained
> calculations.
> I want to compare the total energy of a system with different magnetic
> moment for certain ions, and find the ground state.
> In order to fix magnetic moment at the starting value, lambda = 1000
> is setup. That's a large value.
> As described in INPUT_PW.html, the penalty energy is introduced at
> this time. I wonder if the comparison make any sense?
>
what the code minimizes is the energy including the penalty cost... This
penalty cost is reported in the output.... I'm not sure however whether
the printed value of the total energy includes or not this term...
I hope someone else can comment on that...

stefano de Gironcoli


> Any reply to any question is appreciated. If you can also guide me to
> the position of source code refering to the question, that's great!
>
> Best regards
>
> Shujun Hu
>
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