Electrostatic corrections: Difference between revisions
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For charged cells or for calculations of molecules and surfaces with a large dipole moment, the energy converges very slowly with respect to the size <math>L</math> of the supercell. Using methods discussed by Makov ''et al.''<ref name="Makov95"/> and Neugebauer ''et al.''<ref name="Neugebauer92"/>, VASP can correct for the leading errors (in many details, we have taken a more general approach, though). | For charged cells or for calculations of molecules and surfaces with a large dipole moment, the energy converges very slowly with respect to the size <math>L</math> of the supercell. Using methods discussed by Makov ''et al.''<ref name="Makov95"/> and Neugebauer ''et al.''<ref name="Neugebauer92"/>, VASP can correct for the leading errors (in many details, we have taken a more general approach, though). | ||
== Suggested combination of tags for electrostatic corrections == | == Suggested combination of tags for electrostatic corrections == | ||
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=== Molecules === | === Molecules === | ||
If the system has a net dipole moment, use the {{TAG|LDIPOL}} tag. The former corrects the energies, while the latter corrects the potential and forces. Optionally, {{TAG|DIPOL}} may be set. | If the system has a net dipole moment, use the {{TAG|LDIPOL}} tag. The former corrects the energies, while the latter corrects the potential and forces. Optionally, {{TAG|DIPOL}} may be set. | ||
== Current limitations == | |||
For the current implementation, there are several restrictions; please read carefully: | |||
* Charged systems: | |||
:Quadrupole corrections are only correct for cubic supercells (this means that the calculated 1/''L''<sup>3</sup> corrections are wrong for charged supercells if the supercell is non-cubic). In addition, we have found empirically that for charged systems with excess electrons ({{TAG|NELECT}}>{{TAG|NELECT}}<sub>neutral</sub>) more reliable results can be obtained if the energy after correction of the linear error (1/''L'') is plotted against 1/''L''<sup>3</sup> to extrapolate results manually for ''L''→∞. This is due to the uncertainties in extracting the quadrupole moment of systems with excess electrons. | |||
* Potential corrections are only possible for orthorhombic cells (at least the direction in which the potential is corrected must be orthogonal to the other two directions). | |||
== Related Tags and Sections == | == Related Tags and Sections == | ||
Revision as of 12:53, 19 October 2023
For charged cells or for calculations of molecules and surfaces with a large dipole moment, the energy converges very slowly with respect to the size of the supercell. Using methods discussed by Makov et al.[1] and Neugebauer et al.[2], VASP can correct for the leading errors (in many details, we have taken a more general approach, though).
Suggested combination of tags for electrostatic corrections
In cases where the system has no net charge and no net dipole moment, no specific tags need to be set and this section can be skipped.
Bulk
If the system has a net dipole or net charge, please follow the recommendations of this wiki page.
Surfaces
If the system has a net dipole moment, a combination of IDIPOL=1,2,3 and LDIPOL tags may be used. The former corrects the energies, while the latter corrects the potential and forces. Optionally, DIPOL may be set. The following options may be used to improve convergence for this case.
1. Use any of these tags only after pre-converging the orbitals without the LDIPOL tag
2. The center of charge should be set in the INCAR file (DIPOL= center of mass)
3. Ensure that the cell is sufficiently large to determine the dipole moment with sufficient accuracy (see DIPOL). If the cell is too small, the charge might slash through the vacuum, causing very slow convergence. Often convergence improves with the size of the supercell.
| Warning: Surface calculations with a net charge result in total energies that do not converge. Relative energies may still be useful. |
Wires
Not implemented.
Molecules
If the system has a net dipole moment, use the LDIPOL tag. The former corrects the energies, while the latter corrects the potential and forces. Optionally, DIPOL may be set.
Current limitations
For the current implementation, there are several restrictions; please read carefully:
- Charged systems:
- Quadrupole corrections are only correct for cubic supercells (this means that the calculated 1/L3 corrections are wrong for charged supercells if the supercell is non-cubic). In addition, we have found empirically that for charged systems with excess electrons (NELECT>NELECTneutral) more reliable results can be obtained if the energy after correction of the linear error (1/L) is plotted against 1/L3 to extrapolate results manually for L→∞. This is due to the uncertainties in extracting the quadrupole moment of systems with excess electrons.
- Potential corrections are only possible for orthorhombic cells (at least the direction in which the potential is corrected must be orthogonal to the other two directions).
Related Tags and Sections
NELECT, EPSILON, DIPOL, IDIPOL, LDIPOL, LMONO, EFIELD