2.1 Hydrogen atom: spin is part of the reference

These are unexecuted teaching inputs and starting models. Original diagrams are schematics, not calculated results. Validate version-specific syntax, licensed or authorized data, numerical convergence and the scientific model before using this workflow.

2.1.1 Model, units and provenance

PW cutoffs and energies use Ry, common force output uses Ry/bohr, and pressure uses kbar. Geometry cards state their coordinate units. Different executables have distinct grammars and time-unit conventions.

Shared inputs, conventions and evidence

Original schematic: Hydrogen atom: spin is part of the reference. No numerical results are claimed.

2.1.2 Unexecuted inputs and explicit deltas

Use the accompanying instructions to identify the parent calculation and placement of every delta; a snippet is not automatically a standalone input. Preserve all blank-line and file-provenance requirements.

2.1.2.1 Input block 1

&CONTROL
 calculation='scf', prefix='h', pseudo_dir='./pseudo', outdir='./scratch/h'
/
&SYSTEM
 ibrav=1, A=16.0, nat=1, ntyp=1, ecutwfc=60, ecutrho=480,
 nspin=2, tot_magnetization=1, occupations='fixed'
/
&ELECTRONS
 conv_thr=1.0d-10, mixing_beta=0.3
/
ATOMIC_SPECIES
H 1.008 H.PBE.UPF
ATOMIC_POSITIONS angstrom
H 8.0 8.0 8.0
K_POINTS gamma

2.1.3 Worked investigation

2.1.3.1 Intuition and prerequisites

An isolated H atom has one valence electron. Forcing a nonmagnetic, doubly occupied picture is inappropriate for its atomic reference. A plane-wave code still places that atom in a periodic cell, so isolation must be demonstrated through cell-size tests. This example uses a PBE H pseudopotential, a cubic 16 Å box and Γ sampling. It does not claim that a finite box is perfectly isolated.

2.1.3.2 Original input

See input block 1 above.

2.1.3.3 Interpretation, checks and exercise

Absolute atomic energy depends on the pseudopotential reference and cannot be compared to an all-electron value without a consistent convention. Monitor energy change with box size, not just visual emptiness. Test cutoff convergence independently: a large vacuum box increases plane-wave cost even though the electron count stays one. Exercise: compare the spin-constrained result with a deliberately inappropriate spin treatment and explain why the resulting “binding energy” would use the wrong dissociation reference.