6.1 Γ-point phonons: curvature, not a decorative frequency list

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.

6.1.1 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: Γ-point phonons: curvature, not a decorative frequency list. No numerical results are claimed.

6.1.2 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.

6.1.2.1 Input block 1

Si Gamma response
&INPUTPH
 prefix='si', outdir='./scratch/si', tr2_ph=1.0d-14,
 fildyn='si.G.dyn', ldisp=.false., epsil=.false.
/
0.0 0.0 0.0

6.1.2.2 Input block 2

&INPUT
 fildyn='si.G.dyn', asr='crystal',
 filout='si.G.modes', filmol='si.G.mold'
/

6.1.3 Worked investigation

6.1.3.1 Intuition and prerequisites

A phonon calculation measures the second-order response around a stationary structure. Loose geometry or electronic convergence can masquerade as soft modes. Start with Si at the converged equilibrium geometry and a fresh tight SCF whose saved files remain accessible. Silicon is nonpolar, making it a clean first Γ-point DFPT case without LO–TO complications.

6.1.3.2 Original PH and DYNMAT inputs

See input block 1 above.

6.1.3.3 Outputs, checks and exercise

A two-atom primitive cell has six branches; at Γ three are acoustic translations in the ideal limit. Animate eigenvectors to understand the optical relative motion. An acoustic correction enforces invariance but must not be used to erase a large physical instability or poor input. Vary SCF threshold, PH threshold, cutoff and k mesh one at a time. Exercise: displace atoms along an optical eigenvector by ±u, evaluate energies, and compare the small-u curvature qualitatively with the DFPT mode after accounting for mass normalization.