CHAPTER 08 · BASICS · CALCULATION TYPES

SCF and NSCF

The division of labor between scf and nscf/bands runs, and how to read the output file.

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QE is not one program but a suite of executables, connected by a pipeline that flows through prefix and outdir. The starting point of that pipeline is the division of labor between scf and nscf.

Division of labor

calculation What it does Density Use
'scf' Converges the charge density self-consistently Produces it The starting point of everything: energies, forces, stress
'nscf' Eigenvalues on a frozen density Reads the scf one Dense-grid DOS, Fermi surfaces
'bands' An nscf variant on an arbitrary k-path Reads the scf one Band structures

nscf and bands read the charge density that scf left in outdir/prefix.save/, so the prefix and outdir must match the scf exactly and the run must happen in the same directory. A mismatch gives cannot open file ... .save/charge-density.dat.

scf (coarse k, converge density) ─→ nscf (dense k, tetrahedra) ─→ dos.x / projwfc.x
                                 └→ bands (k-path)             ─→ bands.x

Typical changes in the nscf step: densify the k-grid, switch occupations='tetrahedra', raise nbnd to cover the conduction bands.

How to read the output (do not skip this)

Check these items in every .out file:

!    total energy              =     -93.45 Ry      ← lines marked "!" are converged values
     estimated scf accuracy    <       1.0E-09 Ry   ← below conv_thr?
     the Fermi energy is       6.2 ev               ← printed for metals
     highest occupied, lowest unoccupied level (ev):← printed for insulators (gap estimate)
     total magnetization       =     4.00 Bohr mag/cell  ← spin runs
     absolute magnetization    =     4.12 Bohr mag/cell  ← large difference means AFM components
     Total force               =     0.001 Ry/au
     convergence has been achieved in  12 iterations
  • Only the total-energy line marked ! is the converged value; the unmarked total energy lines are intermediate.
  • The gap between total and absolute magnetization is physical information. For FM they nearly coincide; for AFM the total is near zero while the absolute stays large (Chapter 12).
  • Insulators print highest occupied, lowest unoccupied level (a gap estimate); metals print the Fermi energy is instead. Which of the two appears is itself a diagnosis. Example E10 uses exactly this line to catch GGA calling FeO a metal.
  • The energy decomposition block (one-electron contribution, hartree contribution, xc contribution, ewald contribution) is useful when hunting anomalies.

The timing breakdown at the bottom (init_run, electrons, c_bands, sum_band) tells you where the time goes, and is the first thing to read when planning parallelization (Chapter 18).

Common mistakes

Running the nscf in a different directory from the scf, or deleting outdir in between. The nscf does not build a density; without the scf products it cannot even start. One more: learning with verbosity='low'. With 'high' the output keeps the symmetry operations, the k-point list, and (for DFT+U) the ns occupation matrices.