DEG THRESHOLD
DEG_THRESHOLD = [real]
| Default: DEG_THRESHOLD | = $2*10^{-3}$ |
Description: DEG_THRESHOLD specifies whether two eigenvalues should be treated as degenerate during clustering of orbitals in the linear response code. DEG_THRESHOLD is specified in units of eV.
This parameter is used to find degenerate clusters of orbitals. It is used within VASP whenever linear-response calculations are performed (either with respect to external fields or with respect to ionic displacements). VASP internally uses an inverse-iteration solver with complex shifting to solve the Sternheimer linear-response equation. If the value is too large, non-degenerate eigenvalue pairs may be detected as degenerate pairs. This can lead to very poor convergence or errorneous results, in particular, for systems with low symmetry. Conversely, too small a value might not lead to a proper detection of all degenerate eigenvalues. For insulating systems, this usually does not cause errors, since the imaginary shift can deal with degenerate eigenvalue/eigenvector pairs.
It is recommended to use tiny values for systems that do not possess any symmetry (say around $10^{-5}). For highly symmetric insulators, results should be largely insensitive to the choice of DEG_THRESHOLD. For metals, degenerate states with partial occupancies must be correctly resolved to obtain highly accurate results (so DEG_THRESHOLD must be set to a value that allows safe detection of degenerate states).
| Tip: If you experience large drifts in the Born effective charges and adding more k-points does not help, try adjusting this parameter. |