4.1 Competing magnetic states in bcc iron

These are unexecuted teaching inputs and starting settings to test. Original figures are schematics, not computed results. Use licensed VASP and PAW data, replace every placeholder, record the executable version and validate convergence.

4.1.1 Model, units and provenance

Use eV for energy, Å for length and eV/Å for force; 1 kbar = 0.1 GPa. State normalization per atom, molecule, primitive cell or simulation cell. Record PAW identifiers, release, ZVAL, ENMAX and permitted hashes; never redistribute POTCAR. SCF convergence addresses the chosen electronic problem; convergence of the target property requires separate tests.

Competing magnetic states in bcc iron — original conceptual schematic

Original schematic. Curves explain concepts; blank data areas await verified learner results. No calculation is claimed.

4.1.2 Worked case: procedure, interpretation and checks

Goal and intuition

A magnetic calculation is an optimization on an energy landscape with potentially several self-consistent solutions. The initial moment is a starting hypothesis, not a guarantee. Net magnetization can be zero because a system is nonmagnetic, or because nonzero local moments cancel. These are different states. Iron provides a direct way to practice state tracking before more difficult magnetic oxides or noncollinear problems.

The introductory objective is to compare several collinear starting states in one common two-atom conventional bcc cell at fixed geometry. This is not an exhaustive search for every possible magnetic phase. Prerequisites: the metallic convergence lesson, knowledge of local versus total magnetization, and a consistent Fe PAW choice. Record whether semicore states are included by the chosen potential rather than assuming all Fe potentials are interchangeable.

Original fixed-geometry model

2.86 Å is a starting lattice parameter only.

Workflow

Prepare separate fresh-start directories for FM-like MAGMOM=3.0 3.0, compensated 3.0 -3.0, and a non-spin-polarized ISPIN=1 reference with MAGMOM removed.

What to inspect

Check whether a supposedly antiferromagnetic initialization retained opposite local moments or collapsed to another state. Check whether “zero net moment” hides substantial local polarization. Projected moments depend on how density is partitioned; they are diagnostics, not uniquely defined observables for every atom. A local-moment sum can differ from a full-cell total because of interstitial contributions and projection conventions.

Restart behavior deserves attention: a MAGMOM line generally does not overwrite an already present magnetization density, but it still affects symmetry handling. A proper new-state search must state whether it starts from atomic densities or a previous charge/wavefunction. If an energy difference is comparable to numerical uncertainty, report the ordering as unresolved rather than announcing a ground state.

Limits and exercise

A two-atom cell admits only a small subset of ordering patterns. Omitting spin–orbit coupling means this exercise cannot determine easy-axis anisotropy. Collinear DFT does not automatically describe thermal spin disorder or all correlated magnetic behavior. In current documentation, combining ISPIN=2 and MAGMOM with LNONCOLLINEAR=.TRUE. causes an error from VASP 6.5.0; noncollinear workflows require their own setup rather than adding one flag to this input.

Exercise: deliver a “seed → final state” map showing initial moments, final local moments, net moment, energy per atom and convergence status. If two seeds reach the same state, connect them to one outcome instead of counting them as distinct phases. Explain why magnetic-state uncertainty can exceed the residual SCF energy change.

4.1.3 Unexecuted inputs and analysis scaffolds

These are unexecuted teaching inputs and starting settings to test. Original figures are schematics, not computed results. Use licensed VASP and PAW data, replace every placeholder, record the executable version and validate convergence.

4.1.3.1 Input block 1

Fe conventional bcc trial cell
2.86
1.0 0.0 0.0
0.0 1.0 0.0
0.0 0.0 1.0
Fe
2
Direct
0.0 0.0 0.0
0.5 0.5 0.5

4.1.3.2 Input block 2

SYSTEM = Fe magnetic-state trial
ENCUT = <ECUT_EV>
PREC = Accurate
EDIFF = 1E-7
ALGO = Normal
LREAL = .FALSE.
ISPIN = 2
MAGMOM = 3.0 3.0
LORBIT = 11
ISMEAR = 1
SIGMA = 0.10
ISYM = 0
IBRION = -1
NSW = 0
# No NUPDOWN: let the total spin polarization relax in this comparison