[QE-users] NV0 Center - Absolute Magnetization

[Brendan Smith]

Hi QE Experts,

Please know that before sending this question, I have done my best to
extensively read the existing body of past forum messages and other QE
related learning materials.

I am currently studying NV centers in bulk diamond, and I have a question
regarding a single NV0 (charge neutral) center I have made in a 3x3x3 (216)
atom supercell of bulk diamond. In this system, we have an odd number of
electrons, leaving a single lone electron (spin-up in this case).
Therefore, I expect that my total magnetization should be 1.0 Bohr
mag/cell, and I see this consistently in my QE outputs. I am comfortable in
thinking that my system is ferromagnetic, based on my deductions from
reading the literature. However, I have some discrepancy when it comes to
the absolute magnetization, and it is here where my question lies.

When I fix the total magnetization to be = 1.0 Bohr mag/cell, and use a
fixed occupations scheme, the absolute magnetization for some reason
converges to a value of 2.11 Bohr mag/cell. I find this to be strange,
since it should be around the value of the total magnetization, which is 1
.0 Bohr mag/cell. What this tells me is that for some reason, perhaps I am
converging to some anti-ferromagnetic solution. Then again, I don't know if
this is even possible given that I am using fixed occupations.

Next, when I use occupations = "smearing", ("cold") and set only a guess
starting magnetization (do not fix total magnetization), I find that I
actually get a value for BOTH the total and absolute magnetization to be
around 1.0 Bohr mag/cell, which is more or less what I expect to see, given
that I believe that my system is rightfully a simple ferromagnetic system.

So my question is, what the heck is going on in the case of fixed
occupations? why am I converging to  solution in which the total
magnetization is 1.0 Bohr mag/cell, and the absolute magnetization is 2.11
Bohr mag/cell? Is there any apparent misunderstanding I may be having in my
analysis? I seek your expertise in this regard.

I have attached my PWscf input and output files. The output file that was
used with smearing is labeled "x0.scf_smear.out", and the output file in
which fixed occupations were used along with setting tot_magnetization is
labeled "x0.scf_fixed.out". The input file is labeled "x0.scf.in". To run
the calculations with the fixed occupations, I just comment out the
smearing part and uncomment the tot_magnetization line.

Thank you for your time in reading my email. I look forward to any
suggestions.

Best,
Brendan A. Smith
Ph.D candidate at the State University of New York at Buffalo
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